Adding upstream version 124.0.1.upstream/124.0.1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

92 files changed, 11855 insertions, 0 deletions

diff --git a/devtools/shared/commands/resource/legacy-listeners/console-messages.js b/devtools/shared/commands/resource/legacy-listeners/console-messages.js
new file mode 100644
index 0000000000..ae3f81b4df
--- /dev/null
+++ b/devtools/shared/commands/resource/legacy-listeners/console-messages.js
@@ -0,0 +1,59 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+const ResourceCommand = require("resource://devtools/shared/commands/resource/resource-command.js");
+
+module.exports = async function ({ targetCommand, targetFront, onAvailable }) {
+  // Allow the top level target unconditionnally.
+  // Also allow frame, but only in content toolbox, i.e. still ignore them in
+  // the context of the browser toolbox as we inspect messages via the process
+  // targets
+  const listenForFrames = targetCommand.descriptorFront.isTabDescriptor;
+
+  // Allow workers when messages aren't dispatched to the main thread.
+  const listenForWorkers =
+    !targetCommand.rootFront.traits
+      .workerConsoleApiMessagesDispatchedToMainThread;
+
+  const acceptTarget =
+    targetFront.isTopLevel ||
+    targetFront.targetType === targetCommand.TYPES.PROCESS ||
+    (targetFront.targetType === targetCommand.TYPES.FRAME && listenForFrames) ||
+    (targetFront.targetType === targetCommand.TYPES.WORKER && listenForWorkers);
+
+  if (!acceptTarget) {
+    return;
+  }
+
+  const webConsoleFront = await targetFront.getFront("console");
+  if (webConsoleFront.isDestroyed()) {
+    return;
+  }
+
+  // Request notifying about new messages
+  await webConsoleFront.startListeners(["ConsoleAPI"]);
+
+  // Fetch already existing messages
+  // /!\ The actor implementation requires to call startListeners(ConsoleAPI) first /!\
+  const { messages } = await webConsoleFront.getCachedMessages(["ConsoleAPI"]);
+
+  for (const message of messages) {
+    message.resourceType = ResourceCommand.TYPES.CONSOLE_MESSAGE;
+  }
+  onAvailable(messages);
+
+  // Forward new message events
+  webConsoleFront.on("consoleAPICall", message => {
+    // Ignore console messages that are cloned from the content process
+    // (they aren't relevant to toolboxes still using legacy listeners)
+    if (message.clonedFromContentProcess) {
+      return;
+    }
+
+    message.resourceType = ResourceCommand.TYPES.CONSOLE_MESSAGE;
+    onAvailable([message]);
+  });
+};
diff --git a/devtools/shared/commands/resource/legacy-listeners/css-changes.js b/devtools/shared/commands/resource/legacy-listeners/css-changes.js
new file mode 100644
index 0000000000..e9f3e17075
--- /dev/null
+++ b/devtools/shared/commands/resource/legacy-listeners/css-changes.js
@@ -0,0 +1,28 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+const ResourceCommand = require("resource://devtools/shared/commands/resource/resource-command.js");
+
+module.exports = async function ({ targetFront, onAvailable }) {
+  if (!targetFront.hasActor("changes")) {
+    return;
+  }
+
+  const changesFront = await targetFront.getFront("changes");
+
+  // Get all changes collected up to this point by the ChangesActor on the server,
+  // then fire each change as "add-change".
+  const changes = await changesFront.allChanges();
+  await onAvailable(changes.map(change => toResource(change)));
+
+  changesFront.on("add-change", change => onAvailable([toResource(change)]));
+};
+
+function toResource(change) {
+  return Object.assign(change, {
+    resourceType: ResourceCommand.TYPES.CSS_CHANGE,
+  });
+}
diff --git a/devtools/shared/commands/resource/legacy-listeners/error-messages.js b/devtools/shared/commands/resource/legacy-listeners/error-messages.js
new file mode 100644
index 0000000000..5ba898c917
--- /dev/null
+++ b/devtools/shared/commands/resource/legacy-listeners/error-messages.js
@@ -0,0 +1,62 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+const ResourceCommand = require("resource://devtools/shared/commands/resource/resource-command.js");
+const { MESSAGE_CATEGORY } = require("resource://devtools/shared/constants.js");
+
+module.exports = async function ({ targetCommand, targetFront, onAvailable }) {
+  // Allow the top level target unconditionnally.
+  // Also allow frame, but only in content toolbox, i.e. still ignore them in
+  // the context of the browser toolbox as we inspect messages via the process
+  // targets
+  // Also ignore workers as they are not supported yet. (see bug 1592584)
+  const listenForFrames = targetCommand.descriptorFront.isTabDescriptor;
+  const isAllowed =
+    targetFront.isTopLevel ||
+    targetFront.targetType === targetCommand.TYPES.PROCESS ||
+    (targetFront.targetType === targetCommand.TYPES.FRAME && listenForFrames);
+
+  if (!isAllowed) {
+    return;
+  }
+
+  const webConsoleFront = await targetFront.getFront("console");
+  if (webConsoleFront.isDestroyed()) {
+    return;
+  }
+
+  // Request notifying about new messages. Here the "PageError" type start listening for
+  // both actual PageErrors (emitted as "pageError" events) as well as LogMessages (
+  // emitted as "logMessage" events). This function only set up the listener on the
+  // webConsoleFront for "pageError".
+  await webConsoleFront.startListeners(["PageError"]);
+
+  // Fetch already existing messages
+  // /!\ The actor implementation requires to call startListeners("PageError") first /!\
+  let { messages } = await webConsoleFront.getCachedMessages(["PageError"]);
+
+  // On server < v79, we're also getting CSS Messages that we need to filter out.
+  messages = messages.filter(
+    message => message.pageError.category !== MESSAGE_CATEGORY.CSS_PARSER
+  );
+
+  messages.forEach(message => {
+    message.resourceType = ResourceCommand.TYPES.ERROR_MESSAGE;
+  });
+  // Cached messages don't have the same shape as live messages,
+  // so we need to transform them.
+  onAvailable(messages);
+
+  webConsoleFront.on("pageError", message => {
+    // On server < v79, we're getting CSS Messages that we need to filter out.
+    if (message.pageError.category === MESSAGE_CATEGORY.CSS_PARSER) {
+      return;
+    }
+
+    message.resourceType = ResourceCommand.TYPES.ERROR_MESSAGE;
+    onAvailable([message]);
+  });
+};
diff --git a/devtools/shared/commands/resource/legacy-listeners/moz.build b/devtools/shared/commands/resource/legacy-listeners/moz.build
new file mode 100644
index 0000000000..6ffb469891
--- /dev/null
+++ b/devtools/shared/commands/resource/legacy-listeners/moz.build
@@ -0,0 +1,14 @@
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+DevToolsModules(
+    "console-messages.js",
+    "css-changes.js",
+    "error-messages.js",
+    "platform-messages.js",
+    "reflow.js",
+    "root-node.js",
+    "source.js",
+    "thread-states.js",
+)
diff --git a/devtools/shared/commands/resource/legacy-listeners/platform-messages.js b/devtools/shared/commands/resource/legacy-listeners/platform-messages.js
new file mode 100644
index 0000000000..729696275e
--- /dev/null
+++ b/devtools/shared/commands/resource/legacy-listeners/platform-messages.js
@@ -0,0 +1,44 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+const ResourceCommand = require("resource://devtools/shared/commands/resource/resource-command.js");
+
+module.exports = async function ({ targetCommand, targetFront, onAvailable }) {
+  // Only allow the top level target and processes.
+  // Frames can be ignored as logMessage are never sent to them anyway.
+  // Also ignore workers as they are not supported yet. (see bug 1592584)
+  const isAllowed =
+    targetFront.isTopLevel ||
+    targetFront.targetType === targetCommand.TYPES.PROCESS;
+  if (!isAllowed) {
+    return;
+  }
+
+  const webConsoleFront = await targetFront.getFront("console");
+  if (webConsoleFront.isDestroyed()) {
+    return;
+  }
+
+  // Request notifying about new messages. Here the "PageError" type start listening for
+  // both actual PageErrors (emitted as "pageError" events) as well as LogMessages (
+  // emitted as "logMessage" events). This function only set up the listener on the
+  // webConsoleFront for "logMessage".
+  await webConsoleFront.startListeners(["PageError"]);
+
+  // Fetch already existing messages
+  // /!\ The actor implementation requires to call startListeners("PageError") first /!\
+  const { messages } = await webConsoleFront.getCachedMessages(["LogMessage"]);
+
+  for (const message of messages) {
+    message.resourceType = ResourceCommand.TYPES.PLATFORM_MESSAGE;
+  }
+  onAvailable(messages);
+
+  webConsoleFront.on("logMessage", message => {
+    message.resourceType = ResourceCommand.TYPES.PLATFORM_MESSAGE;
+    onAvailable([message]);
+  });
+};
diff --git a/devtools/shared/commands/resource/legacy-listeners/reflow.js b/devtools/shared/commands/resource/legacy-listeners/reflow.js
new file mode 100644
index 0000000000..63802f510d
--- /dev/null
+++ b/devtools/shared/commands/resource/legacy-listeners/reflow.js
@@ -0,0 +1,24 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+const ResourceCommand = require("resource://devtools/shared/commands/resource/resource-command.js");
+
+module.exports = async function ({ targetFront, onAvailable }) {
+  if (!targetFront.getTrait("isBrowsingContext")) {
+    // The reflows only work with BrowsingContext targets
+    return;
+  }
+  const reflowFront = await targetFront.getFront("reflow");
+  reflowFront.on("reflows", reflows =>
+    onAvailable([
+      {
+        resourceType: ResourceCommand.TYPES.REFLOW,
+        reflows,
+      },
+    ])
+  );
+  await reflowFront.start();
+};
diff --git a/devtools/shared/commands/resource/legacy-listeners/root-node.js b/devtools/shared/commands/resource/legacy-listeners/root-node.js
new file mode 100644
index 0000000000..6fa2bcbf22
--- /dev/null
+++ b/devtools/shared/commands/resource/legacy-listeners/root-node.js
@@ -0,0 +1,61 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+const ResourceCommand = require("resource://devtools/shared/commands/resource/resource-command.js");
+
+module.exports = async function ({ targetFront, onAvailable, onDestroyed }) {
+  // XXX: When watching root node for a non top-level target, this will also
+  // ensure the inspector & walker fronts for the target are initialized.
+  // This also implies that we call reparentRemoteFrame on the new walker, which
+  // will create the link between the parent frame NodeFront and the inner
+  // document NodeFront.
+  //
+  // This is not something that will work when the resource is moved to the
+  // server. When it becomes a server side resource, a RootNode would be emitted
+  // directly by the target actor.
+  //
+  // This probably means that the root node resource cannot remain a NodeFront.
+  // It should not be a front and the client should be responsible for
+  // retrieving the corresponding NodeFront.
+  //
+  // The other thing that we are missing with this patch is that we should only
+  // create inspector & walker fronts (and call reparentRemoteFrame) when we get
+  // a RootNode which is directly under an iframe node which is currently
+  // visible and tracked in the markup view.
+  //
+  // For instance, with the following markup:
+  //   html
+  //     body
+  //       div
+  //         iframe
+  //           remote doc
+  //
+  // If the markup view only sees nodes down to `div`, then the client is not
+  // currently tracking the nodeFront for the `iframe`, and getting a new root
+  // node for the remote document should NOT force the iframe to be tracked on
+  // on the client.
+  //
+  // When we get a RootNode resource, we will need a way to check this before
+  // initializing & reparenting the walker.
+  //
+  if (!targetFront.getTrait("isBrowsingContext")) {
+    // The root-node resource is only available on browsing-context targets.
+    return;
+  }
+
+  const inspectorFront = await targetFront.getFront("inspector");
+  inspectorFront.walker.on("root-available", node => {
+    node.resourceType = ResourceCommand.TYPES.ROOT_NODE;
+    return onAvailable([node]);
+  });
+
+  inspectorFront.walker.on("root-destroyed", node => {
+    node.resourceType = ResourceCommand.TYPES.ROOT_NODE;
+    return onDestroyed([node]);
+  });
+
+  await inspectorFront.walker.watchRootNode();
+};
diff --git a/devtools/shared/commands/resource/legacy-listeners/source.js b/devtools/shared/commands/resource/legacy-listeners/source.js
new file mode 100644
index 0000000000..45ee62f70f
--- /dev/null
+++ b/devtools/shared/commands/resource/legacy-listeners/source.js
@@ -0,0 +1,88 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+const ResourceCommand = require("resource://devtools/shared/commands/resource/resource-command.js");
+
+/**
+ * Emit SOURCE resources, which represents a Javascript source and has the following attributes set on "available":
+ *
+ * - introductionType {null|String}: A string indicating how this source code was introduced into the system.
+ *   This will typically be set to "scriptElement", "eval", ...
+ *   But this may have many other values:
+ *     https://searchfox.org/mozilla-central/rev/ac142717cc067d875e83e4b1316f004f6e063a46/dom/script/ScriptLoader.cpp#2628-2639
+ *     https://searchfox.org/mozilla-central/search?q=symbol:_ZN2JS14CompileOptions19setIntroductionTypeEPKc&redirect=false
+ *     https://searchfox.org/mozilla-central/rev/ac142717cc067d875e83e4b1316f004f6e063a46/devtools/server/actors/source.js#160-169
+ * - sourceMapBaseURL {String}: Base URL where to look for a source map.
+ *   This isn't the source map URL.
+ * - sourceMapURL {null|String}: URL of the source map, if there is one.
+ * - url {null|String}: URL of the source, if it relates to a particular URL.
+ *   Evaled sources won't have any related URL.
+ * - isBlackBoxed {Boolean}: Specifying whether the source actor's 'black-boxed' flag is set.
+ * - extensionName {null|String}: If the source comes from an add-on, the add-on name.
+ */
+module.exports = async function ({ targetCommand, targetFront, onAvailable }) {
+  const isBrowserToolbox =
+    targetCommand.descriptorFront.isBrowserProcessDescriptor;
+  const isNonTopLevelFrameTarget =
+    !targetFront.isTopLevel &&
+    targetFront.targetType === targetCommand.TYPES.FRAME;
+
+  if (isBrowserToolbox && isNonTopLevelFrameTarget) {
+    // In the BrowserToolbox, non-top-level frame targets are already
+    // debugged via content-process targets.
+    return;
+  }
+
+  const threadFront = await targetFront.getFront("thread");
+
+  // Use a list of all notified SourceFront as we don't have a newSource event for all sources
+  // but we sometime get sources notified both via newSource event *and* sources() method...
+  // We store actor ID instead of SourceFront as it appears that multiple SourceFront for the same
+  // actor are created...
+  const sourcesActorIDCache = new Set();
+
+  // Forward new sources (but also existing ones, see next comment)
+  threadFront.on("newSource", ({ source }) => {
+    if (sourcesActorIDCache.has(source.actor)) {
+      return;
+    }
+    sourcesActorIDCache.add(source.actor);
+    // source is a SourceActor's form, add the resourceType attribute on it
+    source.resourceType = ResourceCommand.TYPES.SOURCE;
+    onAvailable([source]);
+  });
+
+  // Forward already existing sources
+  // Note that calling `sources()` will end up emitting `newSource` event for all existing sources.
+  // But not in some cases, for example, when the thread is already paused.
+  // (And yes, it means that already existing sources can be transfered twice over the wire)
+  //
+  // Also, browser_ext_devtools_inspectedWindow_targetSwitch.js creates many top level targets,
+  // for which the SourceMapURLService will fetch sources. But these targets are destroyed while
+  // the test is running and when they are, we purge all pending requests, including this one.
+  // So ignore any error if this request failed on destruction.
+  let sources;
+  try {
+    sources = await threadFront.sources();
+  } catch (e) {
+    if (threadFront.isDestroyed()) {
+      return;
+    }
+    throw e;
+  }
+
+  // Note that `sources()` doesn't encapsulate SourceFront into a `source` attribute
+  // while `newSource` event does.
+  sources = sources.filter(source => {
+    return !sourcesActorIDCache.has(source.actor);
+  });
+  for (const source of sources) {
+    sourcesActorIDCache.add(source.actor);
+    // source is a SourceActor's form, add the resourceType attribute on it
+    source.resourceType = ResourceCommand.TYPES.SOURCE;
+  }
+  onAvailable(sources);
+};
diff --git a/devtools/shared/commands/resource/legacy-listeners/thread-states.js b/devtools/shared/commands/resource/legacy-listeners/thread-states.js
new file mode 100644
index 0000000000..42c922072a
--- /dev/null
+++ b/devtools/shared/commands/resource/legacy-listeners/thread-states.js
@@ -0,0 +1,81 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+const ResourceCommand = require("resource://devtools/shared/commands/resource/resource-command.js");
+
+module.exports = async function ({ targetCommand, targetFront, onAvailable }) {
+  const isBrowserToolbox =
+    targetCommand.descriptorFront.isBrowserProcessDescriptor;
+  const isNonTopLevelFrameTarget =
+    !targetFront.isTopLevel &&
+    targetFront.targetType === targetCommand.TYPES.FRAME;
+
+  if (isBrowserToolbox && isNonTopLevelFrameTarget) {
+    // In the BrowserToolbox, non-top-level frame targets are already
+    // debugged via content-process targets.
+    return;
+  }
+
+  // Wait for the thread actor to be attached, otherwise getFront(thread) will throw for worker targets
+  // This is because worker target are still kind of descriptors and are only resolved into real target
+  // after being attached. And the thread actor ID is only retrieved and available after being attached.
+  await targetFront.onThreadAttached;
+
+  if (targetFront.isDestroyed()) {
+    return;
+  }
+  const threadFront = await targetFront.getFront("thread");
+
+  let isInterrupted = false;
+  const onPausedPacket = packet => {
+    // If paused by an explicit interrupt, which are generated by the
+    // slow script dialog and internal events such as setting
+    // breakpoints, ignore the event.
+    const { why } = packet;
+    if (why.type === "interrupted" && !why.onNext) {
+      isInterrupted = true;
+      return;
+    }
+
+    // Ignore attached events because they are not useful to the user.
+    if (why.type == "alreadyPaused" || why.type == "attached") {
+      return;
+    }
+
+    onAvailable([
+      {
+        resourceType: ResourceCommand.TYPES.THREAD_STATE,
+        state: "paused",
+        why,
+        frame: packet.frame,
+      },
+    ]);
+  };
+  threadFront.on("paused", onPausedPacket);
+
+  threadFront.on("resumed", packet => {
+    // NOTE: the client suppresses resumed events while interrupted
+    // to prevent unintentional behavior.
+    // see [client docs](devtools/client/debugger/src/client/README.md#interrupted) for more information.
+    if (isInterrupted) {
+      isInterrupted = false;
+      return;
+    }
+
+    onAvailable([
+      {
+        resourceType: ResourceCommand.TYPES.THREAD_STATE,
+        state: "resumed",
+      },
+    ]);
+  });
+
+  // Notify about already paused thread
+  const pausedPacket = threadFront.getLastPausePacket();
+  if (pausedPacket) {
+    onPausedPacket(pausedPacket);
+  }
+};
diff --git a/devtools/shared/commands/resource/moz.build b/devtools/shared/commands/resource/moz.build
new file mode 100644
index 0000000000..190589df4b
--- /dev/null
+++ b/devtools/shared/commands/resource/moz.build
@@ -0,0 +1,15 @@
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+DIRS += [
+    "legacy-listeners",
+    "transformers",
+]
+
+DevToolsModules(
+    "resource-command.js",
+)
+
+if CONFIG["MOZ_BUILD_APP"] != "mobile/android":
+    BROWSER_CHROME_MANIFESTS += ["tests/browser.toml"]
diff --git a/devtools/shared/commands/resource/resource-command.js b/devtools/shared/commands/resource/resource-command.js
new file mode 100644
index 0000000000..c45dc6a584
--- /dev/null
+++ b/devtools/shared/commands/resource/resource-command.js
@@ -0,0 +1,1367 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+const { throttle } = require("resource://devtools/shared/throttle.js");
+
+let gLastResourceId = 0;
+
+function cacheKey(resourceType, resourceId) {
+  return `${resourceType}:${resourceId}`;
+}
+
+class ResourceCommand {
+  /**
+   * This class helps retrieving existing and listening to resources.
+   * A resource is something that:
+   *  - the target you are debugging exposes
+   *  - can be created as early as the process/worker/page starts loading
+   *  - can already exist, or will be created later on
+   *  - doesn't require any user data to be fetched, only a type/category
+   *
+   * @param object commands
+   *        The commands object with all interfaces defined from devtools/shared/commands/
+   */
+  constructor({ commands }) {
+    this.targetCommand = commands.targetCommand;
+
+    // Public attribute set by tests to disable throttling
+    this.throttlingDisabled = false;
+
+    this._onTargetAvailable = this._onTargetAvailable.bind(this);
+    this._onTargetDestroyed = this._onTargetDestroyed.bind(this);
+
+    this._onResourceAvailable = this._onResourceAvailable.bind(this);
+    this._onResourceDestroyed = this._onResourceDestroyed.bind(this);
+
+    // Array of all the currently registered watchers, which contains object with attributes:
+    // - {String} resources: list of all resource watched by this one watcher
+    // - {Function} onAvailable: watcher's function to call when a new resource is available
+    // - {Function} onUpdated: watcher's function to call when a resource has been updated
+    // - {Function} onDestroyed: watcher's function to call when a resource is destroyed
+    this._watchers = [];
+
+    // Set of watchers currently going through watchResources, only used to handle
+    // early calls to unwatchResources. Using a Set instead of an array for easier
+    // delete operations.
+    this._pendingWatchers = new Set();
+
+    // Caches for all resources by the order that the resource was taken.
+    this._cache = new Map();
+    this._listenedResources = new Set();
+
+    // WeakMap used to avoid starting a legacy listener twice for the same
+    // target + resource-type pair. Legacy listener creation can be subject to
+    // race conditions.
+    // Maps a target front to an array of resource types.
+    this._existingLegacyListeners = new WeakMap();
+    this._processingExistingResources = new Set();
+
+    // List of targetFront event listener unregistration functions keyed by target front.
+    // These are called when unwatching resources, so if a consumer starts watching resources again,
+    // we don't have listeners registered twice.
+    this._offTargetFrontListeners = new Map();
+
+    this._notifyWatchers = this._notifyWatchers.bind(this);
+    this._throttledNotifyWatchers = throttle(this._notifyWatchers, 100);
+  }
+
+  get watcherFront() {
+    return this.targetCommand.watcherFront;
+  }
+
+  addResourceToCache(resource) {
+    const { resourceId, resourceType } = resource;
+    this._cache.set(cacheKey(resourceType, resourceId), resource);
+  }
+
+  /**
+   * Clear all the resources related to specifed resource types.
+   * Should also trigger clearing of the caches that exists on the related
+   * serverside resource watchers.
+   *
+   * @param {Array:string} resourceTypes
+   *                       A list of all the resource types whose
+   *                       resources shouled be cleared.
+   */
+  async clearResources(resourceTypes) {
+    if (!Array.isArray(resourceTypes)) {
+      throw new Error("clearResources expects a list of resources types");
+    }
+    // Clear the cached resources of the type.
+    for (const [key, resource] of this._cache) {
+      if (resourceTypes.includes(resource.resourceType)) {
+        // NOTE: To anyone paranoid like me, yes it is okay to delete from a Map while iterating it.
+        this._cache.delete(key);
+      }
+    }
+
+    const resourcesToClear = resourceTypes.filter(resourceType =>
+      this.hasResourceCommandSupport(resourceType)
+    );
+    if (resourcesToClear.length) {
+      this.watcherFront.clearResources(resourcesToClear);
+    }
+  }
+  /**
+   * Return all specified resources cached in this watcher.
+   *
+   * @param {String} resourceType
+   * @return {Array} resources cached in this watcher
+   */
+  getAllResources(resourceType) {
+    const result = [];
+    for (const resource of this._cache.values()) {
+      if (resource.resourceType === resourceType) {
+        result.push(resource);
+      }
+    }
+    return result;
+  }
+
+  /**
+   * Return the specified resource cached in this watcher.
+   *
+   * @param {String} resourceType
+   * @param {String} resourceId
+   * @return {Object} resource cached in this watcher
+   */
+  getResourceById(resourceType, resourceId) {
+    return this._cache.get(cacheKey(resourceType, resourceId));
+  }
+
+  /**
+   * Request to start retrieving all already existing instances of given
+   * type of resources and also start watching for the one to be created after.
+   *
+   * @param {Array:string} resources
+   *        List of all resources which should be fetched and observed.
+   * @param {Object} options
+   *        - {Function} onAvailable: This attribute is mandatory.
+   *                                  Function which will be called with an array of resources
+   *                                  each time resource(s) are created.
+   *                                  A second dictionary argument with `areExistingResources` boolean
+   *                                  attribute helps knowing if that's live resources, or some coming
+   *                                  from ResourceCommand cache.
+   *        - {Function} onUpdated:   This attribute is optional.
+   *                                  Function which will be called with an array of updates resources
+   *                                  each time resource(s) are updated.
+   *                                  These resources were previously notified via onAvailable.
+   *        - {Function} onDestroyed: This attribute is optional.
+   *                                  Function which will be called with an array of deleted resources
+   *                                  each time resource(s) are destroyed.
+   *        - {boolean} ignoreExistingResources:
+   *                                  This attribute is optional. Default value is false.
+   *                                  If set to true, onAvailable won't be called with
+   *                                  existing resources.
+   */
+  async watchResources(resources, options) {
+    const {
+      onAvailable,
+      onUpdated,
+      onDestroyed,
+      ignoreExistingResources = false,
+    } = options;
+
+    if (typeof onAvailable !== "function") {
+      throw new Error(
+        "ResourceCommand.watchResources expects an onAvailable function as argument"
+      );
+    }
+
+    for (const type of resources) {
+      if (!this._isValidResourceType(type)) {
+        throw new Error(
+          `ResourceCommand.watchResources invoked with an unknown type: "${type}"`
+        );
+      }
+    }
+
+    // Pending watchers are used in unwatchResources to remove watchers which
+    // are not fully registered yet. Store `onAvailable` which is the unique key
+    // for a watcher, as well as the resources array, so that unwatchResources
+    // can update the array if we stop watching a specific resource.
+    const pendingWatcher = {
+      resources,
+      onAvailable,
+    };
+    this._pendingWatchers.add(pendingWatcher);
+
+    // Bug 1675763: Watcher actor is not available in all situations yet.
+    if (!this._listenerRegistered && this.watcherFront) {
+      this._listenerRegistered = true;
+      // Resources watched from the parent process will be emitted on the Watcher Actor.
+      // So that we also have to listen for this event on it, in addition to all targets.
+      this.watcherFront.on(
+        "resource-available-form",
+        this._onResourceAvailable.bind(this, {
+          watcherFront: this.watcherFront,
+        })
+      );
+      this.watcherFront.on(
+        "resource-updated-form",
+        this._onResourceUpdated.bind(this, { watcherFront: this.watcherFront })
+      );
+      this.watcherFront.on(
+        "resource-destroyed-form",
+        this._onResourceDestroyed.bind(this, {
+          watcherFront: this.watcherFront,
+        })
+      );
+    }
+
+    const promises = [];
+    for (const resource of resources) {
+      promises.push(this._startListening(resource));
+    }
+    await Promise.all(promises);
+
+    // The resource cache is immediately filled when receiving the sources, but they are
+    // emitted with a delay due to throttling. Since the cache can contain resources that
+    // will soon be emitted, we have to flush it before adding the new listeners.
+    // Otherwise _forwardExistingResources might emit resources that will also be emitted by
+    // the next `_notifyWatchers` call done when calling `_startListening`, which will pull the
+    // "already existing" resources.
+    this._notifyWatchers();
+
+    // Update the _pendingWatchers set before adding the watcher to _watchers.
+    this._pendingWatchers.delete(pendingWatcher);
+
+    // If unwatchResources was called in the meantime, use pendingWatcher's
+    // resources to get the updated list of watched resources.
+    const watchedResources = pendingWatcher.resources;
+
+    // If no resource needs to be watched anymore, do not add an empty watcher
+    // to _watchers, and do not notify about cached resources.
+    if (!watchedResources.length) {
+      return;
+    }
+
+    // Register the watcher just after calling _startListening in order to avoid it being called
+    // for already existing resources, which will optionally be notified via _forwardExistingResources
+    this._watchers.push({
+      resources: watchedResources,
+      onAvailable,
+      onUpdated,
+      onDestroyed,
+      pendingEvents: [],
+    });
+
+    if (!ignoreExistingResources) {
+      await this._forwardExistingResources(watchedResources, onAvailable);
+    }
+  }
+
+  /**
+   * Stop watching for given type of resources.
+   * See `watchResources` for the arguments as both methods receive the same.
+   * Note that `onUpdated` and `onDestroyed` attributes of `options` aren't used here.
+   * Only `onAvailable` attribute is looked up and we unregister all the other registered callbacks
+   * when a matching available callback is found.
+   */
+  unwatchResources(resources, options) {
+    const { onAvailable } = options;
+
+    if (typeof onAvailable !== "function") {
+      throw new Error(
+        "ResourceCommand.unwatchResources expects an onAvailable function as argument"
+      );
+    }
+
+    for (const type of resources) {
+      if (!this._isValidResourceType(type)) {
+        throw new Error(
+          `ResourceCommand.unwatchResources invoked with an unknown type: "${type}"`
+        );
+      }
+    }
+
+    // Unregister the callbacks from the watchers registries.
+    // Check _watchers for the fully initialized watchers, as well as
+    // `_pendingWatchers` for new watchers still being created by `watchResources`
+    const allWatchers = [...this._watchers, ...this._pendingWatchers];
+    for (const watcherEntry of allWatchers) {
+      // onAvailable is the only mandatory argument which ends up being used to match
+      // the right watcher entry.
+      if (watcherEntry.onAvailable == onAvailable) {
+        // Remove all resources that we stop watching. We may still watch for some others.
+        watcherEntry.resources = watcherEntry.resources.filter(resourceType => {
+          return !resources.includes(resourceType);
+        });
+      }
+    }
+    this._watchers = this._watchers.filter(entry => {
+      // Remove entries entirely if it isn't watching for any resource type
+      return !!entry.resources.length;
+    });
+
+    // Stop listening to all resources for which we removed the last watcher
+    for (const resource of resources) {
+      const isResourceWatched = allWatchers.some(watcherEntry =>
+        watcherEntry.resources.includes(resource)
+      );
+
+      // Also check in _listenedResources as we may call unwatchResources
+      // for resources that we haven't started watching for.
+      if (!isResourceWatched && this._listenedResources.has(resource)) {
+        this._stopListening(resource);
+      }
+    }
+
+    // Stop watching for targets if we removed the last listener.
+    if (this._listenedResources.size == 0) {
+      this._unwatchAllTargets();
+    }
+  }
+
+  /**
+   * Wait for a single resource of the provided resourceType.
+   *
+   * @param {String} resourceType
+   *        One of ResourceCommand.TYPES, type of the expected resource.
+   * @param {Object} additional options
+   *        - {Boolean} ignoreExistingResources: ignore existing resources or not.
+   *        - {Function} predicate: if provided, will wait until a resource makes
+   *          predicate(resource) return true.
+   * @return {Promise<Object>}
+   *         Return a promise which resolves once we fully settle the resource listener.
+   *         You should await for its resolution before doing the action which may fire
+   *         your resource.
+   *         This promise will expose an object with `onResource` attribute,
+   *         itself being a promise, which will resolve once a matching resource is received.
+   */
+  async waitForNextResource(
+    resourceType,
+    { ignoreExistingResources = false, predicate } = {}
+  ) {
+    // If no predicate was provided, convert to boolean to avoid resolving for
+    // empty `resources` arrays.
+    predicate = predicate || (resource => !!resource);
+
+    let resolve;
+    const promise = new Promise(r => (resolve = r));
+    const onAvailable = async resources => {
+      const matchingResource = resources.find(resource => predicate(resource));
+      if (matchingResource) {
+        this.unwatchResources([resourceType], { onAvailable });
+        resolve(matchingResource);
+      }
+    };
+
+    await this.watchResources([resourceType], {
+      ignoreExistingResources,
+      onAvailable,
+    });
+    return { onResource: promise };
+  }
+
+  /**
+   * Check if there are any watchers for the specified resource.
+   *
+   * @param {String} resourceType
+   *         One of ResourceCommand.TYPES
+   * @return {Boolean}
+   *         If the resources type is beibg watched.
+   */
+  isResourceWatched(resourceType) {
+    return this._listenedResources.has(resourceType);
+  }
+
+  /**
+   * Start watching for all already existing and future targets.
+   *
+   * We are using ALL_TYPES, but this won't force listening to all types.
+   * It will only listen for types which are defined by `TargetCommand.startListening`.
+   */
+  async _watchAllTargets() {
+    if (!this._watchTargetsPromise) {
+      // If this is the very first listener registered, of all kind of resource types:
+      // * we want to start observing targets via TargetCommand
+      // * _onTargetAvailable will be called for each already existing targets and the next one to come
+      this._watchTargetsPromise = this.targetCommand.watchTargets({
+        types: this.targetCommand.ALL_TYPES,
+        onAvailable: this._onTargetAvailable,
+        onDestroyed: this._onTargetDestroyed,
+      });
+    }
+    return this._watchTargetsPromise;
+  }
+
+  _unwatchAllTargets() {
+    if (!this._watchTargetsPromise) {
+      return;
+    }
+
+    for (const offList of this._offTargetFrontListeners.values()) {
+      offList.forEach(off => off());
+    }
+    this._offTargetFrontListeners.clear();
+
+    this._watchTargetsPromise = null;
+    this.targetCommand.unwatchTargets({
+      types: this.targetCommand.ALL_TYPES,
+      onAvailable: this._onTargetAvailable,
+      onDestroyed: this._onTargetDestroyed,
+    });
+  }
+
+  /**
+   * For a given resource type, start the legacy listeners for all already existing targets.
+   * Do that only if we have to. If this resourceType requires legacy listeners.
+   */
+  async _startLegacyListenersForExistingTargets(resourceType) {
+    // If we were already listening to targets, we want to start the legacy listeners
+    // for all already existing targets.
+    //
+    // Only try instantiating the legacy listener, if this resource type:
+    //   - has legacy listener implementation
+    // (new resource types may not be supported by old runtime and just not be received without breaking anything)
+    //   - isn't supported by the server, or, the target type requires the a legacy listener implementation.
+    const shouldRunLegacyListeners =
+      resourceType in LegacyListeners &&
+      (!this.hasResourceCommandSupport(resourceType) ||
+        this._shouldRunLegacyListenerEvenWithWatcherSupport(resourceType));
+    if (shouldRunLegacyListeners) {
+      const promises = [];
+      const targets = this.targetCommand.getAllTargets(
+        this.targetCommand.ALL_TYPES
+      );
+      for (const targetFront of targets) {
+        // We disable warning in case we already registered the legacy listener for this target
+        // as this code may race with the call from onTargetAvailable if we end up having multiple
+        // calls to _startListening in parallel.
+        promises.push(
+          this._watchResourcesForTarget({
+            targetFront,
+            resourceType,
+            disableWarning: true,
+          })
+        );
+      }
+      await Promise.all(promises);
+    }
+  }
+
+  /**
+   * Method called by the TargetCommand for each already existing or target which has just been created.
+   *
+   * @param {Object} arg
+   * @param {Front} arg.targetFront
+   *        The Front of the target that is available.
+   *        This Front inherits from TargetMixin and is typically
+   *        composed of a WindowGlobalTargetFront or ContentProcessTargetFront.
+   * @param {Boolean} arg.isTargetSwitching
+   *         true when the new target was created because of a target switching.
+   */
+  async _onTargetAvailable({ targetFront, isTargetSwitching }) {
+    const resources = [];
+    if (isTargetSwitching) {
+      // WatcherActor currently only watches additional frame targets and
+      // explicitely ignores top level one that may be created when navigating
+      // to a new process.
+      // In order to keep working resources that are being watched via the
+      // Watcher actor, we have to unregister and re-register the resource
+      // types. This will force calling `Resources.watchResources` on the new top
+      // level target.
+      for (const resourceType of Object.values(ResourceCommand.TYPES)) {
+        // ...which has at least one listener...
+        if (!this._listenedResources.has(resourceType)) {
+          continue;
+        }
+
+        if (this._shouldRestartListenerOnTargetSwitching(resourceType)) {
+          this._stopListening(resourceType, {
+            bypassListenerCount: true,
+          });
+          resources.push(resourceType);
+        }
+      }
+    }
+
+    if (targetFront.isDestroyed()) {
+      return;
+    }
+
+    // If we are target switching, we already stop & start listening to all the
+    // currently monitored resources.
+    if (!isTargetSwitching) {
+      // For each resource type...
+      for (const resourceType of Object.values(ResourceCommand.TYPES)) {
+        // ...which has at least one listener...
+        if (!this._listenedResources.has(resourceType)) {
+          continue;
+        }
+        // ...request existing resource and new one to come from this one target
+        // *but* only do that for backward compat, where we don't have the watcher API
+        // (See bug 1626647)
+        await this._watchResourcesForTarget({ targetFront, resourceType });
+      }
+    }
+
+    // Compared to the TargetCommand and Watcher.watchTargets,
+    // We do call Watcher.watchResources, but the events are fired on the target.
+    // That's because the Watcher runs in the parent process/main thread, while resources
+    // are available from the target's process/thread.
+    const offResourceAvailable = targetFront.on(
+      "resource-available-form",
+      this._onResourceAvailable.bind(this, { targetFront })
+    );
+    const offResourceUpdated = targetFront.on(
+      "resource-updated-form",
+      this._onResourceUpdated.bind(this, { targetFront })
+    );
+    const offResourceDestroyed = targetFront.on(
+      "resource-destroyed-form",
+      this._onResourceDestroyed.bind(this, { targetFront })
+    );
+
+    const offList = this._offTargetFrontListeners.get(targetFront) || [];
+    offList.push(
+      offResourceAvailable,
+      offResourceUpdated,
+      offResourceDestroyed
+    );
+
+    if (isTargetSwitching) {
+      await Promise.all(
+        resources.map(resourceType =>
+          this._startListening(resourceType, {
+            bypassListenerCount: true,
+          })
+        )
+      );
+    }
+
+    // DOCUMENT_EVENT's will-navigate should replace target actor's will-navigate event,
+    // but only for targets provided by the watcher actor.
+    // Emit a fake DOCUMENT_EVENT's "will-navigate" out of target actor's will-navigate
+    // until watcher actor is supported by all descriptors (bug 1675763).
+    if (!this.targetCommand.hasTargetWatcherSupport()) {
+      const offWillNavigate = targetFront.on(
+        "will-navigate",
+        ({ url, isFrameSwitching }) => {
+          targetFront.emit("resource-available-form", [
+            {
+              resourceType: this.TYPES.DOCUMENT_EVENT,
+              name: "will-navigate",
+              time: Date.now(), // will-navigate was not passing any timestamp
+              isFrameSwitching,
+              newURI: url,
+            },
+          ]);
+        }
+      );
+      offList.push(offWillNavigate);
+    }
+
+    this._offTargetFrontListeners.set(targetFront, offList);
+  }
+
+  _shouldRestartListenerOnTargetSwitching(resourceType) {
+    // Note that we aren't using isServerTargetSwitchingEnabled, nor checking the
+    // server side target switching preference as we may have server side targets
+    // even when this is false/disabled.
+    // This will happen for bfcache navigations, even with server side targets disabled.
+    // `followWindowGlobalLifeCycle` will be false for the first top level target
+    // and only become true when doing a bfcache navigation.
+    // (only server side targets follow the WindowGlobal lifecycle)
+    // When server side targets are enabled, this will always be true.
+    const isServerSideTarget =
+      this.targetCommand.targetFront.targetForm.followWindowGlobalLifeCycle;
+    if (isServerSideTarget) {
+      // For top-level targets created from the server, only restart legacy
+      // listeners.
+      return !this.hasResourceCommandSupport(resourceType);
+    }
+
+    // For top-level targets created from the client we should always restart
+    // listeners.
+    return true;
+  }
+
+  /**
+   * Method called by the TargetCommand when a target has just been destroyed
+   * @param {Object} arg
+   * @param {Front} arg.targetFront
+   *        The Front of the target that was destroyed
+   * @param {Boolean} arg.isModeSwitching
+   *         true when this is called as the result of a change to the devtools.browsertoolbox.scope pref.
+   */
+  _onTargetDestroyed({ targetFront, isModeSwitching }) {
+    // Clear the map of legacy listeners for this target.
+    this._existingLegacyListeners.set(targetFront, []);
+    this._offTargetFrontListeners.delete(targetFront);
+
+    // Purge the cache from any resource related to the destroyed target.
+    // Top level BrowsingContext target will be purge via DOCUMENT_EVENT will-navigate events.
+    // If we were to clean resources from target-destroyed, we will clear resources
+    // happening between will-navigate and target-destroyed. Typically the navigation request
+    // At the moment, isModeSwitching can only be true when targetFront.isTopLevel isn't true,
+    // so we don't need to add a specific check for isModeSwitching.
+    if (!targetFront.isTopLevel || !targetFront.isBrowsingContext) {
+      for (const [key, resource] of this._cache) {
+        if (resource.targetFront === targetFront) {
+          // NOTE: To anyone paranoid like me, yes it is okay to delete from a Map while iterating it.
+          this._cache.delete(key);
+        }
+      }
+    }
+
+    // Purge "available" pendingEvents for resources from the destroyed target when switching
+    // mode as we want to ignore those.
+    if (isModeSwitching) {
+      for (const watcherEntry of this._watchers) {
+        for (const pendingEvent of watcherEntry.pendingEvents) {
+          if (pendingEvent.callbackType == "available") {
+            pendingEvent.updates = pendingEvent.updates.filter(
+              update => update.targetFront !== targetFront
+            );
+          }
+        }
+      }
+    }
+  }
+
+  /**
+   * Method called either by:
+   * - the backward compatibility code (LegacyListeners)
+   * - target actors RDP events
+   * whenever an already existing resource is being listed or when a new one
+   * has been created.
+   *
+   * @param {Object} source
+   *        A dictionary object with only one of these two attributes:
+   *        - targetFront: a Target Front, if the resource is watched from the target process or thread
+   *        - watcherFront: a Watcher Front, if the resource is watched from the parent process
+   * @param {Array<json/Front>} resources
+   *        Depending on the resource Type, it can be an Array composed of either JSON objects or Fronts,
+   *        which describes the resource.
+   */
+  async _onResourceAvailable({ targetFront, watcherFront }, resources) {
+    let includesDocumentEventWillNavigate = false;
+    let includesDocumentEventDomLoading = false;
+    for (let resource of resources) {
+      const { resourceType } = resource;
+
+      if (watcherFront) {
+        targetFront = await this._getTargetForWatcherResource(resource);
+        // When we receive resources from the Watcher actor,
+        // there is no guarantee that the target front is fully initialized.
+        // The Target Front is initialized by the TargetCommand, by calling TargetFront.attachAndInitThread.
+        // We have to wait for its completion as resources watchers are expecting it to be completed.
+        //
+        // But when navigating, we may receive resources packets for a destroyed target.
+        // Or, in the context of the browser toolbox, they may not relate to any target.
+        if (targetFront) {
+          await targetFront.initialized;
+        }
+      }
+
+      // isAlreadyExistingResource indicates that the resources already existed before
+      // the resource command started watching for this type of resource.
+      resource.isAlreadyExistingResource =
+        this._processingExistingResources.has(resourceType);
+
+      // Put the targetFront on the resource for easy retrieval.
+      // (Resources from the legacy listeners may already have the attribute set)
+      if (!resource.targetFront) {
+        resource.targetFront = targetFront;
+      }
+
+      if (ResourceTransformers[resourceType]) {
+        resource = ResourceTransformers[resourceType]({
+          resource,
+          targetCommand: this.targetCommand,
+          targetFront,
+          watcherFront: this.watcherFront,
+        });
+      }
+
+      if (!resource.resourceId) {
+        resource.resourceId = `auto:${++gLastResourceId}`;
+      }
+
+      // Only consider top level document, and ignore remote iframes top document
+      const isWillNavigate =
+        resourceType == ResourceCommand.TYPES.DOCUMENT_EVENT &&
+        resource.name == "will-navigate";
+      if (isWillNavigate && resource.targetFront.isTopLevel) {
+        includesDocumentEventWillNavigate = true;
+        this._onWillNavigate(resource.targetFront);
+      }
+
+      if (
+        resourceType == ResourceCommand.TYPES.DOCUMENT_EVENT &&
+        resource.name == "dom-loading" &&
+        resource.targetFront.isTopLevel
+      ) {
+        includesDocumentEventDomLoading = true;
+      }
+
+      this._queueResourceEvent("available", resourceType, resource);
+
+      // Avoid storing will-navigate resource and consider it as a transcient resource.
+      // We do that to prevent leaking this resource (and its target) on navigation.
+      // We do clear the cache in _onWillNavigate, that we call a few lines before this.
+      if (!isWillNavigate) {
+        this.addResourceToCache(resource);
+      }
+    }
+
+    // If we receive the DOCUMENT_EVENT for:
+    // - will-navigate
+    // - dom-loading + we're using the service worker legacy listener
+    // then flush immediately the resources to notify about the navigation sooner than later.
+    // (this is especially useful for tests, even if they should probably avoid depending on this...)
+    if (
+      includesDocumentEventWillNavigate ||
+      (includesDocumentEventDomLoading &&
+        !this.targetCommand.hasTargetWatcherSupport("service_worker")) ||
+      this.throttlingDisabled
+    ) {
+      this._notifyWatchers();
+    } else {
+      this._throttledNotifyWatchers();
+    }
+  }
+
+  /**
+   * Method called either by:
+   * - the backward compatibility code (LegacyListeners)
+   * - target actors RDP events
+   * Called everytime a resource is updated in the remote target.
+   *
+   * @param {Object} source
+   *        Please see _onResourceAvailable for this parameter.
+   * @param {Array<Object>} updates
+   *        Depending on the listener.
+   *
+   *        Among the element in the array, the following attributes are given special handling.
+   *          - resourceType {String}:
+   *            The type of resource to be updated.
+   *          - resourceId {String}:
+   *            The id of resource to be updated.
+   *          - resourceUpdates {Object}:
+   *            If resourceUpdates is in the element, a cached resource specified by resourceType
+   *            and resourceId is updated by Object.assign(cachedResource, resourceUpdates).
+   *          - nestedResourceUpdates {Object}:
+   *            If `nestedResourceUpdates` is passed, update one nested attribute with a new value
+   *            This allows updating one attribute of an object stored in a resource's attribute,
+   *            as well as adding new elements to arrays.
+   *            `path` is an array mentioning all nested attribute to walk through.
+   *            `value` is the new nested attribute value to set.
+   *
+   *        And also, the element is passed to the listener as it is as “update” object.
+   *        So if we don't want to update a cached resource but have information want to
+   *        pass on to the listener, can pass it on using attributes other than the ones
+   *        listed above.
+   *        For example, if the element consists of like
+   *        "{ resourceType:… resourceId:…, testValue: “test”, }”,
+   *        the listener can receive the value as follows.
+   *
+   *        onResourceUpdate({ update }) {
+   *          console.log(update.testValue); // “test” should be displayed
+   *        }
+   */
+  async _onResourceUpdated({ targetFront, watcherFront }, updates) {
+    for (const update of updates) {
+      const {
+        resourceType,
+        resourceId,
+        resourceUpdates,
+        nestedResourceUpdates,
+      } = update;
+
+      if (!resourceId) {
+        console.warn(`Expected resource ${resourceType} to have a resourceId`);
+      }
+
+      // See _onResourceAvailable()
+      // We also need to wait for the related targetFront to be initialized
+      // otherwise we would notify about the udpate *before* the available
+      // and the resource won't be in _cache.
+      if (watcherFront) {
+        targetFront = await this._getTargetForWatcherResource(update);
+        // When we receive the navigation request, the target front has already been
+        // destroyed, but this is fine. The cached resource has the reference to
+        // the (destroyed) target front and it is fully initialized.
+        if (targetFront) {
+          await targetFront.initialized;
+        }
+      }
+
+      const existingResource = this._cache.get(
+        cacheKey(resourceType, resourceId)
+      );
+      if (!existingResource) {
+        continue;
+      }
+
+      if (resourceUpdates) {
+        Object.assign(existingResource, resourceUpdates);
+      }
+
+      if (nestedResourceUpdates) {
+        for (const { path, value } of nestedResourceUpdates) {
+          let target = existingResource;
+
+          for (let i = 0; i < path.length - 1; i++) {
+            target = target[path[i]];
+          }
+
+          target[path[path.length - 1]] = value;
+        }
+      }
+      this._queueResourceEvent("updated", resourceType, {
+        resource: existingResource,
+        update,
+      });
+    }
+    this._throttledNotifyWatchers();
+  }
+
+  /**
+   * Called everytime a resource is destroyed in the remote target.
+   * See _onResourceAvailable for the argument description.
+   */
+  async _onResourceDestroyed({ targetFront, watcherFront }, resources) {
+    for (const resource of resources) {
+      const { resourceType, resourceId } = resource;
+      this._cache.delete(cacheKey(resourceType, resourceId));
+      if (!resource.targetFront) {
+        resource.targetFront = targetFront;
+      }
+      this._queueResourceEvent("destroyed", resourceType, resource);
+    }
+    this._throttledNotifyWatchers();
+  }
+
+  _queueResourceEvent(callbackType, resourceType, update) {
+    for (const { resources, pendingEvents } of this._watchers) {
+      // This watcher doesn't listen to this type of resource
+      if (!resources.includes(resourceType)) {
+        continue;
+      }
+      // If we receive a new event of the same type, accumulate the new update in the last event
+      if (pendingEvents.length) {
+        const lastEvent = pendingEvents[pendingEvents.length - 1];
+        if (lastEvent.callbackType == callbackType) {
+          lastEvent.updates.push(update);
+          continue;
+        }
+      }
+      // Otherwise, pile up a new event, which will force calling watcher
+      // callback a new time
+      pendingEvents.push({
+        callbackType,
+        updates: [update],
+      });
+    }
+  }
+
+  /**
+   * Flush the pending event and notify all the currently registered watchers
+   * about all the available, updated and destroyed events that have been accumulated in
+   * `_watchers`'s `pendingEvents` arrays.
+   */
+  _notifyWatchers() {
+    for (const watcherEntry of this._watchers) {
+      const { onAvailable, onUpdated, onDestroyed, pendingEvents } =
+        watcherEntry;
+      // Immediately clear the buffer in order to avoid possible races, where an event listener
+      // would end up somehow adding a new throttled resource
+      watcherEntry.pendingEvents = [];
+
+      for (const { callbackType, updates } of pendingEvents) {
+        try {
+          if (callbackType == "available") {
+            onAvailable(updates, { areExistingResources: false });
+          } else if (callbackType == "updated" && onUpdated) {
+            onUpdated(updates);
+          } else if (callbackType == "destroyed" && onDestroyed) {
+            onDestroyed(updates);
+          }
+        } catch (e) {
+          console.error(
+            "Exception while calling a ResourceCommand",
+            callbackType,
+            "callback",
+            ":",
+            e
+          );
+        }
+      }
+    }
+  }
+
+  // Compute the target front if the resource comes from the Watcher Actor.
+  // (`targetFront` will be null as the watcher is in the parent process
+  // and targets are in distinct processes)
+  _getTargetForWatcherResource(resource) {
+    const { browsingContextID, innerWindowId, resourceType } = resource;
+
+    // Some privileged resources aren't related to any BrowsingContext
+    // and so aren't bound to any Target Front.
+    // Server watchers should pass an explicit "-1" value in order to prevent
+    // silently ignoring an undefined browsingContextID attribute.
+    if (browsingContextID == -1) {
+      return null;
+    }
+
+    if (innerWindowId && this.targetCommand.isServerTargetSwitchingEnabled()) {
+      return this.watcherFront.getWindowGlobalTargetByInnerWindowId(
+        innerWindowId
+      );
+    } else if (browsingContextID) {
+      return this.watcherFront.getWindowGlobalTarget(browsingContextID);
+    }
+    console.error(
+      `Resource of ${resourceType} is missing a browsingContextID or innerWindowId attribute`
+    );
+    return null;
+  }
+
+  _onWillNavigate(targetFront) {
+    // Special case for toolboxes debugging a document,
+    // purge the cache entirely when we start navigating to a new document.
+    // Other toolboxes and additional target for remote iframes or content process
+    // will be purge from onTargetDestroyed.
+
+    // NOTE: we could `clear` the cache here, but technically if anything is
+    // currently iterating over resources provided by getAllResources, that
+    // would interfere with their iteration. We just assign a new Map here to
+    // leave those iterators as is.
+    this._cache = new Map();
+  }
+
+  /**
+   * Tells if the server supports listening to the given resource type
+   * via the watcher actor's watchResources method.
+   *
+   * @return {Boolean} True, if the server supports this type.
+   */
+  hasResourceCommandSupport(resourceType) {
+    return this.watcherFront?.traits?.resources?.[resourceType];
+  }
+
+  /**
+   * Tells if the server supports listening to the given resource type
+   * via the watcher actor's watchResources method, and that, for a specific
+   * target.
+   *
+   * @return {Boolean} True, if the server supports this type.
+   */
+  _hasResourceCommandSupportForTarget(resourceType, targetFront) {
+    // First check if the watcher supports this target type.
+    // If it doesn't, no resource type can be listened via the Watcher actor for this target.
+    if (!this.targetCommand.hasTargetWatcherSupport(targetFront.targetType)) {
+      return false;
+    }
+
+    return this.hasResourceCommandSupport(resourceType);
+  }
+
+  _isValidResourceType(type) {
+    return this.ALL_TYPES.includes(type);
+  }
+
+  /**
+   * Start listening for a given type of resource.
+   * For backward compatibility code, we register the legacy listeners on
+   * each individual target
+   *
+   * @param {String} resourceType
+   *        One string of ResourceCommand.TYPES, which designates the types of resources
+   *        to be listened.
+   * @param {Object}
+   *        - {Boolean} bypassListenerCount
+   *          Pass true to avoid checking/updating the listenersCount map.
+   *          Exclusively used when target switching, to stop & start listening
+   *          to all resources.
+   */
+  async _startListening(resourceType, { bypassListenerCount = false } = {}) {
+    if (!bypassListenerCount) {
+      if (this._listenedResources.has(resourceType)) {
+        return;
+      }
+      this._listenedResources.add(resourceType);
+    }
+
+    this._processingExistingResources.add(resourceType);
+
+    // Ensuring enabling listening to targets.
+    // This will be a no-op expect for the very first call to `_startListening`,
+    // where it is going to call `onTargetAvailable` for all already existing targets,
+    // as well as for those who will be created later.
+    //
+    // Do this *before* calling WatcherActor.watchResources in order to register "resource-available"
+    // listeners on targets before these events start being emitted.
+    await this._watchAllTargets(resourceType);
+
+    // When we are calling _startListening for the first time, _watchAllTargets
+    // will register legacylistener when it will call onTargetAvailable for all existing targets.
+    // But for any next calls to _startListening, _watchAllTargets will be a no-op,
+    // and nothing will start legacy listener for each already registered targets.
+    await this._startLegacyListenersForExistingTargets(resourceType);
+
+    // If the server supports the Watcher API and the Watcher supports
+    // this resource type, use this API
+    if (this.hasResourceCommandSupport(resourceType)) {
+      await this.watcherFront.watchResources([resourceType]);
+    }
+    this._processingExistingResources.delete(resourceType);
+  }
+
+  /**
+   * Return true if the resource should be watched via legacy listener,
+   * even when watcher supports this resource type.
+   *
+   * Bug 1678385: In order to support watching for JS Source resource
+   * for service workers and parent process workers, which aren't supported yet
+   * by the watcher actor, we do not bail out here and allow to execute
+   * the legacy listener for these targets.
+   * Once bug 1608848 is fixed, we can remove this and never trigger
+   * the legacy listeners codepath for these resource types.
+   *
+   * If this isn't fixed soon, we may add other resources we want to see
+   * being fetched from these targets.
+   */
+  _shouldRunLegacyListenerEvenWithWatcherSupport(resourceType) {
+    return WORKER_RESOURCE_TYPES.includes(resourceType);
+  }
+
+  async _forwardExistingResources(resourceTypes, onAvailable) {
+    const existingResources = [];
+    for (const resource of this._cache.values()) {
+      if (resourceTypes.includes(resource.resourceType)) {
+        existingResources.push(resource);
+      }
+    }
+    if (existingResources.length) {
+      await onAvailable(existingResources, { areExistingResources: true });
+    }
+  }
+
+  /**
+   * Call backward compatibility code from `LegacyListeners` in order to listen for a given
+   * type of resource from a given target.
+   */
+  async _watchResourcesForTarget({
+    targetFront,
+    resourceType,
+    disableWarning = false,
+  }) {
+    if (this._hasResourceCommandSupportForTarget(resourceType, targetFront)) {
+      // This resource / target pair should already be handled by the watcher,
+      // no need to start legacy listeners.
+      return;
+    }
+
+    // All workers target types are still not supported by the watcher
+    // so that we have to spawn legacy listener for all their resources.
+    // But some resources are irrelevant to workers, like network events.
+    // And we removed the related legacy listener as they are no longer used.
+    if (
+      targetFront.targetType.endsWith("worker") &&
+      !WORKER_RESOURCE_TYPES.includes(resourceType)
+    ) {
+      return;
+    }
+
+    if (targetFront.isDestroyed()) {
+      return;
+    }
+
+    const onAvailable = this._onResourceAvailable.bind(this, { targetFront });
+    const onUpdated = this._onResourceUpdated.bind(this, { targetFront });
+    const onDestroyed = this._onResourceDestroyed.bind(this, { targetFront });
+
+    if (!(resourceType in LegacyListeners)) {
+      throw new Error(`Missing legacy listener for ${resourceType}`);
+    }
+
+    const legacyListeners =
+      this._existingLegacyListeners.get(targetFront) || [];
+    if (legacyListeners.includes(resourceType)) {
+      if (!disableWarning) {
+        console.warn(
+          `Already started legacy listener for ${resourceType} on ${targetFront.actorID}`
+        );
+      }
+      return;
+    }
+    this._existingLegacyListeners.set(
+      targetFront,
+      legacyListeners.concat(resourceType)
+    );
+
+    try {
+      await LegacyListeners[resourceType]({
+        targetCommand: this.targetCommand,
+        targetFront,
+        onAvailable,
+        onDestroyed,
+        onUpdated,
+      });
+    } catch (e) {
+      // Swallow the error to avoid breaking calls to watchResources which will
+      // loop on all existing targets to create legacy listeners.
+      // If a legacy listener fails to handle a target for some reason, we
+      // should still try to process other targets as much as possible.
+      // See Bug 1687645.
+      console.error(
+        `Failed to start [${resourceType}] legacy listener for target ${targetFront.actorID}`,
+        e
+      );
+    }
+  }
+
+  /**
+   * Reverse of _startListening. Stop listening for a given type of resource.
+   * For backward compatibility, we unregister from each individual target.
+   *
+   * See _startListening for parameters description.
+   */
+  _stopListening(resourceType, { bypassListenerCount = false } = {}) {
+    if (!bypassListenerCount) {
+      if (!this._listenedResources.has(resourceType)) {
+        throw new Error(
+          `Stopped listening for resource '${resourceType}' that isn't being listened to`
+        );
+      }
+      this._listenedResources.delete(resourceType);
+    }
+
+    // Clear the cached resources of the type.
+    for (const [key, resource] of this._cache) {
+      if (resource.resourceType == resourceType) {
+        // NOTE: To anyone paranoid like me, yes it is okay to delete from a Map while iterating it.
+        this._cache.delete(key);
+      }
+    }
+
+    // If the server supports the Watcher API and the Watcher supports
+    // this resource type, use this API
+    if (this.hasResourceCommandSupport(resourceType)) {
+      if (!this.watcherFront.isDestroyed()) {
+        this.watcherFront.unwatchResources([resourceType]);
+      }
+
+      const shouldRunLegacyListeners =
+        this._shouldRunLegacyListenerEvenWithWatcherSupport(resourceType);
+      if (!shouldRunLegacyListeners) {
+        return;
+      }
+    }
+    // Otherwise, fallback on backward compat mode and use LegacyListeners.
+
+    // If this was the last listener, we should stop watching these events from the actors
+    // and the actors should stop watching things from the platform
+    const targets = this.targetCommand.getAllTargets(
+      this.targetCommand.ALL_TYPES
+    );
+    for (const target of targets) {
+      this._unwatchResourcesForTarget(target, resourceType);
+    }
+  }
+
+  /**
+   * Backward compatibility code, reverse of _watchResourcesForTarget.
+   */
+  _unwatchResourcesForTarget(targetFront, resourceType) {
+    if (this._hasResourceCommandSupportForTarget(resourceType, targetFront)) {
+      // This resource / target pair should already be handled by the watcher,
+      // no need to stop legacy listeners.
+    }
+    // Is there really a point in:
+    // - unregistering `onAvailable` RDP event callbacks from target-scoped actors?
+    // - calling `stopListeners()` as we are most likely closing the toolbox and destroying everything?
+    //
+    // It is important to keep this method synchronous and do as less as possible
+    // in the case of toolbox destroy.
+    //
+    // We are aware of one case where that might be useful.
+    // When a panel is disabled via the options panel, after it has been opened.
+    // Would that justify doing this? Is there another usecase?
+
+    // XXX: This is most likely only needed to avoid growing the Map infinitely.
+    // Unless in the "disabled panel" use case mentioned in the comment above,
+    // we should not see the same target actorID again.
+    const listeners = this._existingLegacyListeners.get(targetFront);
+    if (listeners && listeners.includes(resourceType)) {
+      const remainingListeners = listeners.filter(l => l !== resourceType);
+      this._existingLegacyListeners.set(targetFront, remainingListeners);
+    }
+  }
+}
+
+ResourceCommand.TYPES = ResourceCommand.prototype.TYPES = {
+  CONSOLE_MESSAGE: "console-message",
+  CSS_CHANGE: "css-change",
+  CSS_MESSAGE: "css-message",
+  CSS_REGISTERED_PROPERTIES: "css-registered-properties",
+  ERROR_MESSAGE: "error-message",
+  PLATFORM_MESSAGE: "platform-message",
+  DOCUMENT_EVENT: "document-event",
+  ROOT_NODE: "root-node",
+  STYLESHEET: "stylesheet",
+  NETWORK_EVENT: "network-event",
+  WEBSOCKET: "websocket",
+  COOKIE: "cookies",
+  LOCAL_STORAGE: "local-storage",
+  SESSION_STORAGE: "session-storage",
+  CACHE_STORAGE: "Cache",
+  EXTENSION_STORAGE: "extension-storage",
+  INDEXED_DB: "indexed-db",
+  NETWORK_EVENT_STACKTRACE: "network-event-stacktrace",
+  REFLOW: "reflow",
+  SOURCE: "source",
+  THREAD_STATE: "thread-state",
+  JSTRACER_TRACE: "jstracer-trace",
+  JSTRACER_STATE: "jstracer-state",
+  SERVER_SENT_EVENT: "server-sent-event",
+  LAST_PRIVATE_CONTEXT_EXIT: "last-private-context-exit",
+};
+ResourceCommand.ALL_TYPES = ResourceCommand.prototype.ALL_TYPES = Object.values(
+  ResourceCommand.TYPES
+);
+module.exports = ResourceCommand;
+
+// This is the list of resource types supported by workers.
+// We need such list to know when forcing to run the legacy listeners
+// and when to avoid try to spawn some unsupported ones for workers.
+const WORKER_RESOURCE_TYPES = [
+  ResourceCommand.TYPES.CONSOLE_MESSAGE,
+  ResourceCommand.TYPES.ERROR_MESSAGE,
+  ResourceCommand.TYPES.SOURCE,
+  ResourceCommand.TYPES.THREAD_STATE,
+];
+
+// Backward compat code for each type of resource.
+// Each section added here should eventually be removed once the equivalent server
+// code is implement in Firefox, in its release channel.
+const LegacyListeners = {
+  async [ResourceCommand.TYPES.DOCUMENT_EVENT]({
+    targetCommand,
+    targetFront,
+    onAvailable,
+  }) {
+    // DocumentEventsListener of webconsole handles only top level document.
+    if (!targetFront.isTopLevel) {
+      return;
+    }
+
+    const webConsoleFront = await targetFront.getFront("console");
+    webConsoleFront.on("documentEvent", event => {
+      event.resourceType = ResourceCommand.TYPES.DOCUMENT_EVENT;
+      onAvailable([event]);
+    });
+    await webConsoleFront.startListeners(["DocumentEvents"]);
+  },
+};
+loader.lazyRequireGetter(
+  LegacyListeners,
+  ResourceCommand.TYPES.CONSOLE_MESSAGE,
+  "resource://devtools/shared/commands/resource/legacy-listeners/console-messages.js"
+);
+loader.lazyRequireGetter(
+  LegacyListeners,
+  ResourceCommand.TYPES.CSS_CHANGE,
+  "resource://devtools/shared/commands/resource/legacy-listeners/css-changes.js"
+);
+loader.lazyRequireGetter(
+  LegacyListeners,
+  ResourceCommand.TYPES.CSS_MESSAGE,
+  "resource://devtools/shared/commands/resource/legacy-listeners/css-messages.js"
+);
+loader.lazyRequireGetter(
+  LegacyListeners,
+  ResourceCommand.TYPES.ERROR_MESSAGE,
+  "resource://devtools/shared/commands/resource/legacy-listeners/error-messages.js"
+);
+loader.lazyRequireGetter(
+  LegacyListeners,
+  ResourceCommand.TYPES.PLATFORM_MESSAGE,
+  "resource://devtools/shared/commands/resource/legacy-listeners/platform-messages.js"
+);
+loader.lazyRequireGetter(
+  LegacyListeners,
+  ResourceCommand.TYPES.ROOT_NODE,
+  "resource://devtools/shared/commands/resource/legacy-listeners/root-node.js"
+);
+
+loader.lazyRequireGetter(
+  LegacyListeners,
+  ResourceCommand.TYPES.SOURCE,
+  "resource://devtools/shared/commands/resource/legacy-listeners/source.js"
+);
+loader.lazyRequireGetter(
+  LegacyListeners,
+  ResourceCommand.TYPES.THREAD_STATE,
+  "resource://devtools/shared/commands/resource/legacy-listeners/thread-states.js"
+);
+
+loader.lazyRequireGetter(
+  LegacyListeners,
+  ResourceCommand.TYPES.REFLOW,
+  "resource://devtools/shared/commands/resource/legacy-listeners/reflow.js"
+);
+
+// Optional transformers for each type of resource.
+// Each module added here should be a function that will receive the resource, the target, …
+// and perform some transformation on the resource before it will be emitted.
+// This is a good place to handle backward compatibility and manual resource marshalling.
+const ResourceTransformers = {};
+
+loader.lazyRequireGetter(
+  ResourceTransformers,
+  ResourceCommand.TYPES.CONSOLE_MESSAGE,
+  "resource://devtools/shared/commands/resource/transformers/console-messages.js"
+);
+loader.lazyRequireGetter(
+  ResourceTransformers,
+  ResourceCommand.TYPES.ERROR_MESSAGE,
+  "resource://devtools/shared/commands/resource/transformers/error-messages.js"
+);
+loader.lazyRequireGetter(
+  ResourceTransformers,
+  ResourceCommand.TYPES.CACHE_STORAGE,
+  "resource://devtools/shared/commands/resource/transformers/storage-cache.js"
+);
+loader.lazyRequireGetter(
+  ResourceTransformers,
+  ResourceCommand.TYPES.COOKIE,
+  "resource://devtools/shared/commands/resource/transformers/storage-cookie.js"
+);
+loader.lazyRequireGetter(
+  ResourceTransformers,
+  ResourceCommand.TYPES.EXTENSION_STORAGE,
+  "resource://devtools/shared/commands/resource/transformers/storage-extension.js"
+);
+loader.lazyRequireGetter(
+  ResourceTransformers,
+  ResourceCommand.TYPES.INDEXED_DB,
+  "resource://devtools/shared/commands/resource/transformers/storage-indexed-db.js"
+);
+loader.lazyRequireGetter(
+  ResourceTransformers,
+  ResourceCommand.TYPES.LOCAL_STORAGE,
+  "resource://devtools/shared/commands/resource/transformers/storage-local-storage.js"
+);
+loader.lazyRequireGetter(
+  ResourceTransformers,
+  ResourceCommand.TYPES.SESSION_STORAGE,
+  "resource://devtools/shared/commands/resource/transformers/storage-session-storage.js"
+);
+loader.lazyRequireGetter(
+  ResourceTransformers,
+  ResourceCommand.TYPES.NETWORK_EVENT,
+  "resource://devtools/shared/commands/resource/transformers/network-events.js"
+);
+loader.lazyRequireGetter(
+  ResourceTransformers,
+  ResourceCommand.TYPES.THREAD_STATE,
+  "resource://devtools/shared/commands/resource/transformers/thread-states.js"
+);
diff --git a/devtools/shared/commands/resource/tests/breakpoint_document.html b/devtools/shared/commands/resource/tests/breakpoint_document.html
new file mode 100644
index 0000000000..2291094646
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/breakpoint_document.html
@@ -0,0 +1,21 @@
+<!DOCTYPE HTML>
+<html>
+  <head>
+    <meta charset="utf8">
+    <title>Test breakpoint document</title>
+    <!-- Any copyright is dedicated to the Public Domain.
+       - http://creativecommons.org/publicdomain/zero/1.0/ -->
+  </head>
+  <body>
+    <script>
+    "use strict";
+    /* eslint-disable */
+    function testFunction() {
+      console.log("test Function ran");
+    }
+    function runDebuggerStatement() {
+      debugger;
+    }
+    </script>
+  </body>
+</html>
diff --git a/devtools/shared/commands/resource/tests/browser.toml b/devtools/shared/commands/resource/tests/browser.toml
new file mode 100644
index 0000000000..def009b710
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/browser.toml
@@ -0,0 +1,128 @@
+[DEFAULT]
+tags = "devtools"
+subsuite = "devtools"
+support-files = [
+  "!/devtools/client/shared/test/shared-head.js",
+  "!/devtools/client/shared/test/telemetry-test-helpers.js",
+  "!/devtools/client/shared/test/highlighter-test-actor.js",
+  "head.js",
+  "breakpoint_document.html",
+  "doc_console.html",
+  "doc_console_iframe.html",
+  "empty.html",
+  "network_document.html",
+  "network_document_navigation.html",
+  "network_navigation.js",
+  "early_console_document.html",
+  "fission_document.html",
+  "fission_document_workers.html",
+  "fission_iframe.html",
+  "fission_iframe_workers.html",
+  "service-worker-sources.js",
+  "sources.html",
+  "sources.js",
+  "sse_backend.sjs",
+  "sse_frontend_iframe.html",
+  "sse_frontend.html",
+  "style_document.css",
+  "style_document.html",
+  "style_iframe.css",
+  "style_iframe.html",
+  "stylesheets-nested-iframes.html",
+  "test_image.png",
+  "test_service_worker.js",
+  "test_worker.js",
+  "websocket_backend_wsh.py",
+  "websocket_frontend_iframe.html",
+  "websocket_frontend.html",
+  "worker-sources.js",
+]
+
+["browser_browser_resources_console_messages.js"]
+
+["browser_resources_clear_resources.js"]
+
+["browser_resources_client_caching.js"]
+
+["browser_resources_console_messages.js"]
+
+["browser_resources_console_messages_navigation.js"]
+
+["browser_resources_console_messages_workers.js"]
+
+["browser_resources_css_changes.js"]
+
+["browser_resources_css_messages.js"]
+
+["browser_resources_css_registered_properties.js"]
+skip-if = ["!fission"]
+
+["browser_resources_document_events.js"]
+skip-if = [
+  "os == 'linux' && bits == 64", # Bug 1715878
+]
+
+["browser_resources_error_messages.js"]
+
+["browser_resources_getAllResources.js"]
+
+["browser_resources_invalid_api_usage.js"]
+
+["browser_resources_last_private_context_exit.js"]
+
+["browser_resources_network_event_stacktraces.js"]
+
+["browser_resources_network_events.js"]
+
+["browser_resources_network_events_cache.js"]
+
+["browser_resources_network_events_navigation.js"]
+
+["browser_resources_network_events_parent_process.js"]
+
+["browser_resources_platform_messages.js"]
+
+["browser_resources_reflows.js"]
+
+["browser_resources_root_node.js"]
+
+["browser_resources_scope_flag.js"]
+
+["browser_resources_server_sent_events.js"]
+
+["browser_resources_several_resources.js"]
+
+["browser_resources_sources.js"]
+skip-if = [
+  "os == 'linux' && bits == 64", # Bug 1744565
+  "win11_2009", # Bug 1767772
+  "apple_catalina", # Bug 1767772
+]
+
+["browser_resources_stylesheets.js"]
+
+["browser_resources_stylesheets_header.js"]
+
+["browser_resources_stylesheets_import.js"]
+
+["browser_resources_stylesheets_navigation.js"]
+
+["browser_resources_stylesheets_nested_iframes.js"]
+
+["browser_resources_target_destroy.js"]
+
+["browser_resources_target_resources_race.js"]
+
+["browser_resources_target_switching.js"]
+
+["browser_resources_thread_states.js"]
+
+["browser_resources_unwatch_early.js"]
+
+["browser_resources_watch_unwatch_multiple.js"]
+
+["browser_resources_websocket.js"]
+skip-if = [
+  "http3", # Bug 1829298
+  "http2",
+]
diff --git a/devtools/shared/commands/resource/tests/browser_browser_resources_console_messages.js b/devtools/shared/commands/resource/tests/browser_browser_resources_console_messages.js
new file mode 100644
index 0000000000..1c6c776e64
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/browser_browser_resources_console_messages.js
@@ -0,0 +1,87 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test the ResourceCommand API around CONSOLE_MESSAGE for the whole browser
+
+const TEST_URL = URL_ROOT_SSL + "early_console_document.html";
+
+add_task(async function () {
+  // Enable Multiprocess Browser Toolbox.
+  await pushPref("devtools.browsertoolbox.scope", "everything");
+
+  const { client, resourceCommand, targetCommand } =
+    await initMultiProcessResourceCommand();
+
+  const d = Date.now();
+  const CACHED_MESSAGE_TEXT = `cached-${d}`;
+  const LIVE_MESSAGE_TEXT = `live-${d}`;
+
+  info(
+    "Log some messages *before* calling ResourceCommand.watchResources in order to " +
+      "assert the behavior of already existing messages."
+  );
+  console.log(CACHED_MESSAGE_TEXT);
+
+  info("Wait for existing browser mochitest log");
+  const { onResource } = await resourceCommand.waitForNextResource(
+    resourceCommand.TYPES.CONSOLE_MESSAGE,
+    {
+      ignoreExistingResources: false,
+      predicate({ message }) {
+        return message.arguments[0] === CACHED_MESSAGE_TEXT;
+      },
+    }
+  );
+  const existingMsg = await onResource;
+  ok(existingMsg, "The existing log was retrieved");
+  is(
+    existingMsg.isAlreadyExistingResource,
+    true,
+    "isAlreadyExistingResource is true for the existing message"
+  );
+
+  const { onResource: onMochitestRuntimeLog } =
+    await resourceCommand.waitForNextResource(
+      resourceCommand.TYPES.CONSOLE_MESSAGE,
+      {
+        ignoreExistingResources: false,
+        predicate({ message }) {
+          return message.arguments[0] === LIVE_MESSAGE_TEXT;
+        },
+      }
+    );
+  console.log(LIVE_MESSAGE_TEXT);
+
+  info("Wait for runtime browser mochitest log");
+  const runtimeLogResource = await onMochitestRuntimeLog;
+  ok(runtimeLogResource, "The runtime log was retrieved");
+  is(
+    runtimeLogResource.isAlreadyExistingResource,
+    false,
+    "isAlreadyExistingResource is false for the runtime message"
+  );
+
+  const { onResource: onEarlyLog } = await resourceCommand.waitForNextResource(
+    resourceCommand.TYPES.CONSOLE_MESSAGE,
+    {
+      ignoreExistingResources: true,
+      predicate({ message }) {
+        return message.arguments[0] === "early-page-log";
+      },
+    }
+  );
+  await addTab(TEST_URL);
+  info("Wait for early page log");
+  const earlyResource = await onEarlyLog;
+  ok(earlyResource, "The early page log was retrieved");
+  is(
+    earlyResource.isAlreadyExistingResource,
+    false,
+    "isAlreadyExistingResource is false for the early message"
+  );
+
+  targetCommand.destroy();
+  await client.close();
+});
diff --git a/devtools/shared/commands/resource/tests/browser_resources_clear_resources.js b/devtools/shared/commands/resource/tests/browser_resources_clear_resources.js
new file mode 100644
index 0000000000..44068cb141
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/browser_resources_clear_resources.js
@@ -0,0 +1,90 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test the clearResources function of the ResourceCommand
+
+add_task(async () => {
+  const tab = await addTab(`${URL_ROOT_SSL}empty.html`);
+  const { client, resourceCommand, targetCommand } = await initResourceCommand(
+    tab
+  );
+
+  info("Assert the initial no of resources");
+  assertNoOfResources(resourceCommand, 0, 0);
+
+  const onAvailable = () => {};
+  const onUpdated = () => {};
+
+  await resourceCommand.watchResources(
+    [
+      resourceCommand.TYPES.CONSOLE_MESSAGE,
+      resourceCommand.TYPES.NETWORK_EVENT,
+    ],
+    { onAvailable, onUpdated }
+  );
+
+  info("Log some messages");
+  await logConsoleMessages(tab.linkedBrowser, ["log1", "log2", "log3"]);
+
+  info("Trigger some network requests");
+  const EXAMPLE_DOMAIN = "https://example.com/";
+  await triggerNetworkRequests(tab.linkedBrowser, [
+    `await fetch("${EXAMPLE_DOMAIN}/request1.html", { method: "GET" });`,
+    `await fetch("${EXAMPLE_DOMAIN}/request2.html", { method: "GET" });`,
+  ]);
+
+  assertNoOfResources(resourceCommand, 3, 2);
+
+  info("Clear the network event resources");
+  await resourceCommand.clearResources([resourceCommand.TYPES.NETWORK_EVENT]);
+  assertNoOfResources(resourceCommand, 3, 0);
+
+  info("Clear the console message resources");
+  await resourceCommand.clearResources([resourceCommand.TYPES.CONSOLE_MESSAGE]);
+  assertNoOfResources(resourceCommand, 0, 0);
+
+  resourceCommand.unwatchResources(
+    [
+      resourceCommand.TYPES.CONSOLE_MESSAGE,
+      resourceCommand.TYPES.NETWORK_EVENT,
+    ],
+    { onAvailable, onUpdated, ignoreExistingResources: true }
+  );
+
+  targetCommand.destroy();
+  await client.close();
+});
+
+function assertNoOfResources(
+  resourceCommand,
+  expectedNoOfConsoleMessageResources,
+  expectedNoOfNetworkEventResources
+) {
+  const actualNoOfConsoleMessageResources = resourceCommand.getAllResources(
+    resourceCommand.TYPES.CONSOLE_MESSAGE
+  ).length;
+  is(
+    actualNoOfConsoleMessageResources,
+    expectedNoOfConsoleMessageResources,
+    `There are ${actualNoOfConsoleMessageResources} console messages resources`
+  );
+
+  const actualNoOfNetworkEventResources = resourceCommand.getAllResources(
+    resourceCommand.TYPES.NETWORK_EVENT
+  ).length;
+  is(
+    actualNoOfNetworkEventResources,
+    expectedNoOfNetworkEventResources,
+    `There are ${actualNoOfNetworkEventResources} network event resources`
+  );
+}
+
+function logConsoleMessages(browser, messages) {
+  return SpecialPowers.spawn(browser, [messages], innerMessages => {
+    for (const message of innerMessages) {
+      content.console.log(message);
+    }
+  });
+}
diff --git a/devtools/shared/commands/resource/tests/browser_resources_client_caching.js b/devtools/shared/commands/resource/tests/browser_resources_client_caching.js
new file mode 100644
index 0000000000..ae398f73cc
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/browser_resources_client_caching.js
@@ -0,0 +1,380 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test the cache mechanism of the ResourceCommand.
+
+const TEST_URI = "data:text/html;charset=utf-8,<!DOCTYPE html>Cache Test";
+
+add_task(async function () {
+  info("Test whether multiple listener can get same cached resources");
+
+  const tab = await addTab(TEST_URI);
+
+  const { client, resourceCommand, targetCommand } = await initResourceCommand(
+    tab
+  );
+
+  info("Add messages as existing resources");
+  const messages = ["a", "b", "c"];
+  await logMessages(tab.linkedBrowser, messages);
+
+  info("Register first listener");
+  const cachedResources1 = [];
+  await resourceCommand.watchResources(
+    [resourceCommand.TYPES.CONSOLE_MESSAGE],
+    {
+      onAvailable(resources, { areExistingResources }) {
+        ok(areExistingResources, "All resources are already existing ones");
+        cachedResources1.push(...resources);
+      },
+    }
+  );
+
+  info("Register second listener");
+  const cachedResources2 = [];
+  await resourceCommand.watchResources(
+    [resourceCommand.TYPES.CONSOLE_MESSAGE],
+    {
+      onAvailable(resources, { areExistingResources }) {
+        ok(areExistingResources, "All resources are already existing ones");
+        cachedResources2.push(...resources);
+      },
+    }
+  );
+
+  assertContents(cachedResources1, messages);
+  assertResources(cachedResources2, cachedResources1);
+
+  targetCommand.destroy();
+  await client.close();
+});
+
+add_task(async function () {
+  info(
+    "Test whether the cache is reflecting existing resources and additional resources"
+  );
+
+  const tab = await addTab(TEST_URI);
+
+  const { client, resourceCommand, targetCommand } = await initResourceCommand(
+    tab
+  );
+
+  info("Add messages as existing resources");
+  const existingMessages = ["a", "b", "c"];
+  await logMessages(tab.linkedBrowser, existingMessages);
+
+  info("Register first listener to get all available resources");
+  const availableResources = [];
+  // We first get notified about existing resources
+  let shouldBeExistingResources = true;
+  await resourceCommand.watchResources(
+    [resourceCommand.TYPES.CONSOLE_MESSAGE],
+    {
+      onAvailable(resources, { areExistingResources }) {
+        is(
+          areExistingResources,
+          shouldBeExistingResources,
+          "areExistingResources flag is correct"
+        );
+        availableResources.push(...resources);
+      },
+    }
+  );
+  // Then, we are notified about, new, live ones
+  shouldBeExistingResources = false;
+
+  info("Add messages as additional resources");
+  const additionalMessages = ["d", "e"];
+  await logMessages(tab.linkedBrowser, additionalMessages);
+
+  info("Wait until onAvailable is called expected times");
+  const allMessages = [...existingMessages, ...additionalMessages];
+  await waitUntil(() => availableResources.length === allMessages.length);
+
+  info("Register second listener to get the cached resources");
+  const cachedResources = [];
+  await resourceCommand.watchResources(
+    [resourceCommand.TYPES.CONSOLE_MESSAGE],
+    {
+      onAvailable(resources, { areExistingResources }) {
+        ok(areExistingResources, "All resources are already existing ones");
+        cachedResources.push(...resources);
+      },
+    }
+  );
+
+  assertContents(availableResources, allMessages);
+  assertResources(cachedResources, availableResources);
+
+  targetCommand.destroy();
+  await client.close();
+});
+
+add_task(async function () {
+  info("Test whether the cache is cleared when navigation");
+
+  const tab = await addTab(TEST_URI);
+
+  const { client, resourceCommand, targetCommand } = await initResourceCommand(
+    tab
+  );
+
+  info("Add messages as existing resources");
+  const existingMessages = ["a", "b", "c"];
+  await logMessages(tab.linkedBrowser, existingMessages);
+
+  info("Register first listener");
+  await resourceCommand.watchResources(
+    [resourceCommand.TYPES.CONSOLE_MESSAGE],
+    {
+      onAvailable: () => {},
+    }
+  );
+
+  info("Reload the page");
+  await BrowserTestUtils.reloadTab(tab);
+
+  info("Register second listener");
+  const cachedResources = [];
+  await resourceCommand.watchResources(
+    [resourceCommand.TYPES.CONSOLE_MESSAGE],
+    {
+      onAvailable: resources => cachedResources.push(...resources),
+    }
+  );
+
+  is(cachedResources.length, 0, "The cache in ResourceCommand is cleared");
+
+  targetCommand.destroy();
+  await client.close();
+});
+
+add_task(async function () {
+  info("Test with multiple resource types");
+
+  const tab = await addTab(TEST_URI);
+
+  const { client, resourceCommand, targetCommand } = await initResourceCommand(
+    tab
+  );
+
+  info("Register first listener to get all available resources");
+  const availableResources = [];
+  await resourceCommand.watchResources(
+    [
+      resourceCommand.TYPES.CONSOLE_MESSAGE,
+      resourceCommand.TYPES.ERROR_MESSAGE,
+    ],
+    {
+      onAvailable: resources => availableResources.push(...resources),
+    }
+  );
+
+  info("Add messages as console message");
+  const consoleMessages1 = ["a", "b", "c"];
+  await logMessages(tab.linkedBrowser, consoleMessages1);
+
+  info("Add message as error message");
+  const errorMessages = ["document.doTheImpossible();"];
+  await triggerErrors(tab.linkedBrowser, errorMessages);
+
+  info("Add messages as console message again");
+  const consoleMessages2 = ["d", "e"];
+  await logMessages(tab.linkedBrowser, consoleMessages2);
+
+  info("Wait until the getting all available resources");
+  const totalResourceCount =
+    consoleMessages1.length + errorMessages.length + consoleMessages2.length;
+  await waitUntil(() => {
+    return availableResources.length === totalResourceCount;
+  });
+
+  info("Register listener to get the cached resources");
+  const cachedResources = [];
+  await resourceCommand.watchResources(
+    [
+      resourceCommand.TYPES.CONSOLE_MESSAGE,
+      resourceCommand.TYPES.ERROR_MESSAGE,
+    ],
+    {
+      onAvailable: resources => cachedResources.push(...resources),
+    }
+  );
+
+  assertResources(cachedResources, availableResources);
+
+  targetCommand.destroy();
+  await client.close();
+});
+
+add_task(async function () {
+  info("Test multiple listeners with/without ignoreExistingResources");
+  await testIgnoreExistingResources(true);
+  await testIgnoreExistingResources(false);
+});
+
+async function testIgnoreExistingResources(isFirstListenerIgnoreExisting) {
+  const tab = await addTab(TEST_URI);
+
+  const { client, resourceCommand, targetCommand } = await initResourceCommand(
+    tab
+  );
+
+  info("Add messages as existing resources");
+  const existingMessages = ["a", "b", "c"];
+  await logMessages(tab.linkedBrowser, existingMessages);
+
+  info("Register first listener");
+  const cachedResources1 = [];
+  await resourceCommand.watchResources(
+    [resourceCommand.TYPES.CONSOLE_MESSAGE],
+    {
+      onAvailable: resources => cachedResources1.push(...resources),
+      ignoreExistingResources: isFirstListenerIgnoreExisting,
+    }
+  );
+
+  info("Register second listener");
+  const cachedResources2 = [];
+  await resourceCommand.watchResources(
+    [resourceCommand.TYPES.CONSOLE_MESSAGE],
+    {
+      onAvailable: resources => cachedResources2.push(...resources),
+      ignoreExistingResources: !isFirstListenerIgnoreExisting,
+    }
+  );
+
+  const cachedResourcesWithFlag = isFirstListenerIgnoreExisting
+    ? cachedResources1
+    : cachedResources2;
+  const cachedResourcesWithoutFlag = isFirstListenerIgnoreExisting
+    ? cachedResources2
+    : cachedResources1;
+
+  info("Check the existing resources both listeners got");
+  assertContents(cachedResourcesWithFlag, []);
+  assertContents(cachedResourcesWithoutFlag, existingMessages);
+
+  info("Add messages as additional resources");
+  const additionalMessages = ["d", "e"];
+  await logMessages(tab.linkedBrowser, additionalMessages);
+
+  info("Wait until onAvailable is called expected times");
+  await waitUntil(
+    () => cachedResourcesWithFlag.length === additionalMessages.length
+  );
+  const allMessages = [...existingMessages, ...additionalMessages];
+  await waitUntil(
+    () => cachedResourcesWithoutFlag.length === allMessages.length
+  );
+
+  info("Check the resources after adding messages");
+  assertContents(cachedResourcesWithFlag, additionalMessages);
+  assertContents(cachedResourcesWithoutFlag, allMessages);
+
+  targetCommand.destroy();
+  await client.close();
+}
+
+add_task(async function () {
+  info("Test that onAvailable is not called with an empty resources array");
+
+  const tab = await addTab(TEST_URI);
+
+  const { client, resourceCommand, targetCommand } = await initResourceCommand(
+    tab
+  );
+
+  info("Register first listener to get all available resources");
+  const availableResources = [];
+  let onAvailableCallCount = 0;
+  const onAvailable = resources => {
+    ok(
+      !!resources.length,
+      "onAvailable is called with a non empty resources array"
+    );
+    availableResources.push(...resources);
+    onAvailableCallCount++;
+  };
+
+  await resourceCommand.watchResources(
+    [resourceCommand.TYPES.CONSOLE_MESSAGE],
+    { onAvailable }
+  );
+  is(availableResources.length, 0, "availableResources array is empty");
+  is(onAvailableCallCount, 0, "onAvailable was never called");
+
+  info("Add messages as console message");
+  await logMessages(tab.linkedBrowser, ["expected message"]);
+
+  await waitUntil(() => availableResources.length === 1);
+  is(availableResources.length, 1, "availableResources array has one item");
+  is(onAvailableCallCount, 1, "onAvailable was called only once");
+  is(
+    availableResources[0].message.arguments[0],
+    "expected message",
+    "onAvailable was called with the expected resource"
+  );
+
+  resourceCommand.unwatchResources([resourceCommand.TYPES.CONSOLE_MESSAGE], {
+    onAvailable,
+  });
+  targetCommand.destroy();
+  await client.close();
+});
+
+function assertContents(resources, expectedMessages) {
+  is(
+    resources.length,
+    expectedMessages.length,
+    "Number of the resources is correct"
+  );
+
+  for (let i = 0; i < expectedMessages.length; i++) {
+    const resource = resources[i];
+    const message = resource.message.arguments[0];
+    const expectedMessage = expectedMessages[i];
+    is(message, expectedMessage, `The ${i}th content is correct`);
+  }
+}
+
+function assertResources(resources, expectedResources) {
+  is(
+    resources.length,
+    expectedResources.length,
+    "Number of the resources is correct"
+  );
+
+  for (let i = 0; i < resources.length; i++) {
+    const resource = resources[i];
+    const expectedResource = expectedResources[i];
+    Assert.strictEqual(
+      resource,
+      expectedResource,
+      `The ${i}th resource is correct`
+    );
+  }
+}
+
+function logMessages(browser, messages) {
+  return ContentTask.spawn(browser, { messages }, args => {
+    for (const message of args.messages) {
+      content.console.log(message);
+    }
+  });
+}
+
+async function triggerErrors(browser, errorScripts) {
+  for (const errorScript of errorScripts) {
+    await ContentTask.spawn(browser, errorScript, expr => {
+      const document = content.document;
+      const container = document.createElement("script");
+      document.body.appendChild(container);
+      container.textContent = expr;
+      container.remove();
+    });
+  }
+}
diff --git a/devtools/shared/commands/resource/tests/browser_resources_console_messages.js b/devtools/shared/commands/resource/tests/browser_resources_console_messages.js
new file mode 100644
index 0000000000..6f02cd5a77
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/browser_resources_console_messages.js
@@ -0,0 +1,623 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test the ResourceCommand API around CONSOLE_MESSAGE
+//
+// Reproduces assertions from: devtools/shared/webconsole/test/chrome/test_cached_messages.html
+// And now more. Once we remove the console actor's startListeners in favor of watcher class
+// We could remove that other old test.
+
+const FISSION_TEST_URL = URL_ROOT_SSL + "fission_document.html";
+const IFRAME_URL = URL_ROOT_ORG_SSL + "fission_iframe.html";
+
+add_task(async function () {
+  info("Execute test in top level document");
+  await testTabConsoleMessagesResources(false);
+  await testTabConsoleMessagesResourcesWithIgnoreExistingResources(false);
+
+  info("Execute test in an iframe document, possibly remote with fission");
+  await testTabConsoleMessagesResources(true);
+  await testTabConsoleMessagesResourcesWithIgnoreExistingResources(true);
+});
+
+async function testTabConsoleMessagesResources(executeInIframe) {
+  const tab = await addTab(FISSION_TEST_URL);
+
+  const { client, resourceCommand, targetCommand } = await initResourceCommand(
+    tab
+  );
+
+  info(
+    "Log some messages *before* calling ResourceCommand.watchResources in order to " +
+      "assert the behavior of already existing messages."
+  );
+  await logExistingMessages(tab.linkedBrowser, executeInIframe);
+
+  const targetDocumentUrl = executeInIframe ? IFRAME_URL : FISSION_TEST_URL;
+
+  let runtimeDoneResolve;
+  const expectedExistingCalls =
+    getExpectedExistingConsoleCalls(targetDocumentUrl);
+  const expectedRuntimeCalls =
+    getExpectedRuntimeConsoleCalls(targetDocumentUrl);
+  const onRuntimeDone = new Promise(resolve => (runtimeDoneResolve = resolve));
+  const onAvailable = resources => {
+    for (const resource of resources) {
+      is(
+        resource.resourceType,
+        resourceCommand.TYPES.CONSOLE_MESSAGE,
+        "Received a message"
+      );
+      ok(resource.message, "message is wrapped into a message attribute");
+      const isCachedMessage = !!expectedExistingCalls.length;
+      const expected = (
+        isCachedMessage ? expectedExistingCalls : expectedRuntimeCalls
+      ).shift();
+      checkConsoleAPICall(resource.message, expected);
+      is(
+        resource.isAlreadyExistingResource,
+        isCachedMessage,
+        "isAlreadyExistingResource has the expected value"
+      );
+
+      if (!expectedRuntimeCalls.length) {
+        runtimeDoneResolve();
+      }
+    }
+  };
+
+  await resourceCommand.watchResources(
+    [resourceCommand.TYPES.CONSOLE_MESSAGE],
+    {
+      onAvailable,
+    }
+  );
+  is(
+    expectedExistingCalls.length,
+    0,
+    "Got the expected number of existing messages"
+  );
+
+  info(
+    "Now log messages *after* the call to ResourceCommand.watchResources and after having received all existing messages"
+  );
+  await logRuntimeMessages(tab.linkedBrowser, executeInIframe);
+
+  info("Waiting for all runtime messages");
+  await onRuntimeDone;
+
+  is(
+    expectedRuntimeCalls.length,
+    0,
+    "Got the expected number of runtime messages"
+  );
+
+  targetCommand.destroy();
+  await client.close();
+}
+
+async function testTabConsoleMessagesResourcesWithIgnoreExistingResources(
+  executeInIframe
+) {
+  info("Test ignoreExistingResources option for console messages");
+  const tab = await addTab(FISSION_TEST_URL);
+
+  const { client, resourceCommand, targetCommand } = await initResourceCommand(
+    tab
+  );
+
+  info(
+    "Check whether onAvailable will not be called with existing console messages"
+  );
+  await logExistingMessages(tab.linkedBrowser, executeInIframe);
+
+  const availableResources = [];
+  await resourceCommand.watchResources(
+    [resourceCommand.TYPES.CONSOLE_MESSAGE],
+    {
+      onAvailable: resources => availableResources.push(...resources),
+      ignoreExistingResources: true,
+    }
+  );
+  is(
+    availableResources.length,
+    0,
+    "onAvailable wasn't called for existing console messages"
+  );
+
+  info(
+    "Check whether onAvailable will be called with the future console messages"
+  );
+  await logRuntimeMessages(tab.linkedBrowser, executeInIframe);
+  const targetDocumentUrl = executeInIframe ? IFRAME_URL : FISSION_TEST_URL;
+  const expectedRuntimeConsoleCalls =
+    getExpectedRuntimeConsoleCalls(targetDocumentUrl);
+  await waitUntil(
+    () => availableResources.length === expectedRuntimeConsoleCalls.length
+  );
+  const expectedTargetFront =
+    executeInIframe && (isFissionEnabled() || isEveryFrameTargetEnabled())
+      ? targetCommand
+          .getAllTargets([targetCommand.TYPES.FRAME])
+          .find(target => target.url == IFRAME_URL)
+      : targetCommand.targetFront;
+  for (let i = 0; i < expectedRuntimeConsoleCalls.length; i++) {
+    const resource = availableResources[i];
+    const { message, targetFront } = resource;
+    is(
+      targetFront,
+      expectedTargetFront,
+      "The targetFront property is the expected one"
+    );
+    const expected = expectedRuntimeConsoleCalls[i];
+    checkConsoleAPICall(message, expected);
+    is(
+      resource.isAlreadyExistingResource,
+      false,
+      "isAlreadyExistingResource is false since we're ignoring existing resources"
+    );
+  }
+
+  targetCommand.destroy();
+  await client.close();
+}
+
+async function logExistingMessages(browser, executeInIframe) {
+  let browsingContext = browser.browsingContext;
+  if (executeInIframe) {
+    browsingContext = await SpecialPowers.spawn(
+      browser,
+      [],
+      function frameScript() {
+        return content.document.querySelector("iframe").browsingContext;
+      }
+    );
+  }
+  return evalInBrowsingContext(browsingContext, function pageScript() {
+    console.log("foobarBaz-log", undefined);
+    console.info("foobarBaz-info", null);
+    console.warn("foobarBaz-warn", document.body);
+  });
+}
+
+/**
+ * Helper function similar to spawn, but instead of executing the script
+ * as a Frame Script, with privileges and including test harness in stacktraces,
+ * execute the script as a regular page script, without privileges and without any
+ * preceding stack.
+ *
+ * @param {BrowsingContext} The browsing context into which the script should be evaluated
+ * @param {Function|String} The JS to execute in the browsing context
+ *
+ * @return {Promise} Which resolves once the JS is done executing in the page
+ */
+function evalInBrowsingContext(browsingContext, script) {
+  return SpecialPowers.spawn(browsingContext, [String(script)], expr => {
+    const document = content.document;
+    const scriptEl = document.createElement("script");
+    document.body.appendChild(scriptEl);
+    // Force the immediate execution of the stringified JS function passed in `expr`
+    scriptEl.textContent = "new " + expr;
+    scriptEl.remove();
+  });
+}
+
+// For both existing and runtime messages, we execute console API
+// from a page script evaluated via evalInBrowsingContext.
+// Records here the function used to execute the script in the page.
+const EXPECTED_FUNCTION_NAME = "pageScript";
+
+const NUMBER_REGEX = /^\d+$/;
+// timeStamp are the result of a number in microsecond divided by 1000.
+// so we can't expect a precise number of decimals, or even if there would
+// be decimals at all.
+const FRACTIONAL_NUMBER_REGEX = /^\d+(\.\d{1,3})?$/;
+
+function getExpectedExistingConsoleCalls(documentFilename) {
+  const defaultProperties = {
+    filename: documentFilename,
+    columnNumber: NUMBER_REGEX,
+    lineNumber: NUMBER_REGEX,
+    timeStamp: FRACTIONAL_NUMBER_REGEX,
+    innerWindowID: NUMBER_REGEX,
+    chromeContext: undefined,
+    counter: undefined,
+    prefix: undefined,
+    private: undefined,
+    stacktrace: undefined,
+    styles: undefined,
+    timer: undefined,
+  };
+
+  return [
+    {
+      ...defaultProperties,
+      level: "log",
+      arguments: ["foobarBaz-log", { type: "undefined" }],
+    },
+    {
+      ...defaultProperties,
+      level: "info",
+      arguments: ["foobarBaz-info", { type: "null" }],
+    },
+    {
+      ...defaultProperties,
+      level: "warn",
+      arguments: ["foobarBaz-warn", { type: "object", actor: /[a-z]/ }],
+    },
+  ];
+}
+
+const longString = new Array(DevToolsServer.LONG_STRING_LENGTH + 2).join("a");
+function getExpectedRuntimeConsoleCalls(documentFilename) {
+  const defaultStackFrames = [
+    // This is the usage of "new " + expr from `evalInBrowsingContext`
+    {
+      filename: documentFilename,
+      lineNumber: NUMBER_REGEX,
+      columnNumber: NUMBER_REGEX,
+    },
+  ];
+
+  const defaultProperties = {
+    filename: documentFilename,
+    columnNumber: NUMBER_REGEX,
+    lineNumber: NUMBER_REGEX,
+    timeStamp: FRACTIONAL_NUMBER_REGEX,
+    innerWindowID: NUMBER_REGEX,
+    chromeContext: undefined,
+    counter: undefined,
+    prefix: undefined,
+    private: undefined,
+    stacktrace: undefined,
+    styles: undefined,
+    timer: undefined,
+  };
+
+  return [
+    {
+      ...defaultProperties,
+      level: "log",
+      arguments: ["foobarBaz-log", { type: "undefined" }],
+    },
+    {
+      ...defaultProperties,
+      level: "log",
+      arguments: ["Float from not a number: NaN"],
+    },
+    {
+      ...defaultProperties,
+      level: "log",
+      arguments: ["Float from string: 1.200000"],
+    },
+    {
+      ...defaultProperties,
+      level: "log",
+      arguments: ["Float from number: 1.300000"],
+    },
+    {
+      ...defaultProperties,
+      level: "log",
+      arguments: ["BigInt 123 and 456"],
+    },
+    {
+      ...defaultProperties,
+      level: "log",
+      arguments: ["message with ", "style"],
+      styles: ["color: blue;", "background: red; font-size: 2em;"],
+    },
+    {
+      ...defaultProperties,
+      level: "info",
+      arguments: ["foobarBaz-info", { type: "null" }],
+    },
+    {
+      ...defaultProperties,
+      level: "warn",
+      arguments: ["foobarBaz-warn", { type: "object", actor: /[a-z]/ }],
+    },
+    {
+      ...defaultProperties,
+      level: "debug",
+      arguments: [{ type: "null" }],
+    },
+    {
+      ...defaultProperties,
+      level: "trace",
+      stacktrace: [
+        {
+          filename: documentFilename,
+          functionName: EXPECTED_FUNCTION_NAME,
+        },
+        ...defaultStackFrames,
+      ],
+    },
+    {
+      ...defaultProperties,
+      level: "dir",
+      arguments: [
+        {
+          type: "object",
+          actor: /[a-z]/,
+          class: "HTMLDocument",
+        },
+        {
+          type: "object",
+          actor: /[a-z]/,
+          class: "Location",
+        },
+      ],
+    },
+    {
+      ...defaultProperties,
+      level: "log",
+      arguments: [
+        "foo",
+        {
+          type: "longString",
+          initial: longString.substring(
+            0,
+            DevToolsServer.LONG_STRING_INITIAL_LENGTH
+          ),
+          length: longString.length,
+          actor: /[a-z]/,
+        },
+      ],
+    },
+    {
+      ...defaultProperties,
+      level: "count",
+      arguments: ["myCounter"],
+      counter: {
+        count: 1,
+        label: "myCounter",
+      },
+    },
+    {
+      ...defaultProperties,
+      level: "count",
+      arguments: ["myCounter"],
+      counter: {
+        count: 2,
+        label: "myCounter",
+      },
+    },
+    {
+      ...defaultProperties,
+      level: "count",
+      arguments: ["default"],
+      counter: {
+        count: 1,
+        label: "default",
+      },
+    },
+    {
+      ...defaultProperties,
+      level: "countReset",
+      arguments: ["myCounter"],
+      counter: {
+        count: 0,
+        label: "myCounter",
+      },
+    },
+    {
+      ...defaultProperties,
+      level: "countReset",
+      arguments: ["unknownCounter"],
+      counter: {
+        error: "counterDoesntExist",
+        label: "unknownCounter",
+      },
+    },
+    {
+      ...defaultProperties,
+      level: "time",
+      arguments: ["myTimer"],
+      timer: {
+        name: "myTimer",
+      },
+    },
+    {
+      ...defaultProperties,
+      level: "time",
+      arguments: ["myTimer"],
+      timer: {
+        name: "myTimer",
+        error: "timerAlreadyExists",
+      },
+    },
+    {
+      ...defaultProperties,
+      level: "timeLog",
+      arguments: ["myTimer"],
+      timer: {
+        name: "myTimer",
+        duration: NUMBER_REGEX,
+      },
+    },
+    {
+      ...defaultProperties,
+      level: "timeEnd",
+      arguments: ["myTimer"],
+      timer: {
+        name: "myTimer",
+        duration: NUMBER_REGEX,
+      },
+    },
+    {
+      ...defaultProperties,
+      level: "time",
+      arguments: ["default"],
+      timer: {
+        name: "default",
+      },
+    },
+    {
+      ...defaultProperties,
+      level: "timeLog",
+      arguments: ["default"],
+      timer: {
+        name: "default",
+        duration: NUMBER_REGEX,
+      },
+    },
+    {
+      ...defaultProperties,
+      level: "timeEnd",
+      arguments: ["default"],
+      timer: {
+        name: "default",
+        duration: NUMBER_REGEX,
+      },
+    },
+    {
+      ...defaultProperties,
+      level: "timeLog",
+      arguments: ["unknownTimer"],
+      timer: {
+        name: "unknownTimer",
+        error: "timerDoesntExist",
+      },
+    },
+    {
+      ...defaultProperties,
+      level: "timeEnd",
+      arguments: ["unknownTimer"],
+      timer: {
+        name: "unknownTimer",
+        error: "timerDoesntExist",
+      },
+    },
+    {
+      ...defaultProperties,
+      level: "error",
+      arguments: ["foobarBaz-asmjs-error", { type: "undefined" }],
+
+      stacktrace: [
+        {
+          filename: documentFilename,
+          functionName: "fromAsmJS",
+        },
+        {
+          filename: documentFilename,
+          functionName: "inAsmJS2",
+        },
+        {
+          filename: documentFilename,
+          functionName: "inAsmJS1",
+        },
+        {
+          filename: documentFilename,
+          functionName: EXPECTED_FUNCTION_NAME,
+        },
+        ...defaultStackFrames,
+      ],
+    },
+    {
+      ...defaultProperties,
+      level: "log",
+      filename:
+        "chrome://mochitests/content/browser/devtools/shared/commands/resource/tests/browser_resources_console_messages.js",
+      arguments: [
+        {
+          type: "object",
+          actor: /[a-z]/,
+          class: "Restricted",
+        },
+      ],
+      chromeContext: true,
+    },
+  ];
+}
+
+async function logRuntimeMessages(browser, executeInIframe) {
+  let browsingContext = browser.browsingContext;
+  if (executeInIframe) {
+    browsingContext = await SpecialPowers.spawn(
+      browser,
+      [],
+      function frameScript() {
+        return content.document.querySelector("iframe").browsingContext;
+      }
+    );
+  }
+  // First inject LONG_STRING_LENGTH in global scope it order to easily use it after
+  await evalInBrowsingContext(
+    browsingContext,
+    `function () {window.LONG_STRING_LENGTH = ${DevToolsServer.LONG_STRING_LENGTH};}`
+  );
+  await evalInBrowsingContext(browsingContext, function pageScript() {
+    const _longString = new Array(window.LONG_STRING_LENGTH + 2).join("a");
+
+    console.log("foobarBaz-log", undefined);
+
+    console.log("Float from not a number: %f", "foo");
+    console.log("Float from string: %f", "1.2");
+    console.log("Float from number: %f", 1.3);
+    console.log("BigInt %d and %i", 123n, 456n);
+    console.log(
+      "%cmessage with %cstyle",
+      "color: blue;",
+      "background: red; font-size: 2em;"
+    );
+
+    console.info("foobarBaz-info", null);
+    console.warn("foobarBaz-warn", document.documentElement);
+    console.debug(null);
+    console.trace();
+    console.dir(document, location);
+    console.log("foo", _longString);
+
+    console.count("myCounter");
+    console.count("myCounter");
+    console.count();
+    console.countReset("myCounter");
+    // will cause warnings because unknownCounter doesn't exist
+    console.countReset("unknownCounter");
+
+    console.time("myTimer");
+    // will cause warning because myTimer already exist
+    console.time("myTimer");
+    console.timeLog("myTimer");
+    console.timeEnd("myTimer");
+    console.time();
+    console.timeLog();
+    console.timeEnd();
+    // // will cause warnings because unknownTimer doesn't exist
+    console.timeLog("unknownTimer");
+    console.timeEnd("unknownTimer");
+
+    function fromAsmJS() {
+      console.error("foobarBaz-asmjs-error", undefined);
+    }
+
+    (function (global, foreign) {
+      "use asm";
+      function inAsmJS2() {
+        foreign.fromAsmJS();
+      }
+      function inAsmJS1() {
+        inAsmJS2();
+      }
+      return inAsmJS1;
+    })(null, { fromAsmJS })();
+  });
+  await SpecialPowers.spawn(browsingContext, [], function frameScript() {
+    const sandbox = new Cu.Sandbox(null, { invisibleToDebugger: true });
+    const sandboxObj = sandbox.eval("new Object");
+    content.console.log(sandboxObj);
+  });
+}
+
+// Copied from devtools/shared/webconsole/test/chrome/common.js
+function checkConsoleAPICall(call, expected) {
+  is(
+    call.arguments?.length || 0,
+    expected.arguments?.length || 0,
+    "number of arguments"
+  );
+
+  checkObject(call, expected);
+}
diff --git a/devtools/shared/commands/resource/tests/browser_resources_console_messages_navigation.js b/devtools/shared/commands/resource/tests/browser_resources_console_messages_navigation.js
new file mode 100644
index 0000000000..3d6fc697da
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/browser_resources_console_messages_navigation.js
@@ -0,0 +1,190 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test the resource command API around CONSOLE_MESSAGE when navigating
+// tab and inner iframes to distinct origin/processes.
+
+const TEST_URL = URL_ROOT_COM_SSL + "doc_console.html";
+const TEST_IFRAME_URL = URL_ROOT_ORG_SSL + "doc_console_iframe.html";
+const TEST_DOMAIN = "https://example.org";
+add_task(async function () {
+  const START_URL = "data:text/html;charset=utf-8,foo";
+  const tab = await addTab(START_URL);
+
+  const { client, resourceCommand, targetCommand } = await initResourceCommand(
+    tab
+  );
+
+  await testCrossProcessTabNavigation(tab.linkedBrowser, resourceCommand);
+  await testCrossProcessIframeNavigation(tab.linkedBrowser, resourceCommand);
+
+  targetCommand.destroy();
+  await client.close();
+  BrowserTestUtils.removeTab(tab);
+});
+
+async function testCrossProcessTabNavigation(browser, resourceCommand) {
+  info(
+    "Navigate the top level document from data: URI to a https document including remote iframes"
+  );
+
+  let doneResolve;
+  const messages = [];
+  const onConsoleLogsComplete = new Promise(resolve => (doneResolve = resolve));
+
+  const onAvailable = resources => {
+    messages.push(...resources);
+    if (messages.length == 2) {
+      doneResolve();
+    }
+  };
+
+  await resourceCommand.watchResources(
+    [resourceCommand.TYPES.CONSOLE_MESSAGE],
+    { onAvailable }
+  );
+
+  const onLoaded = BrowserTestUtils.browserLoaded(browser);
+  BrowserTestUtils.startLoadingURIString(browser, TEST_URL);
+  await onLoaded;
+
+  info("Wait for log message");
+  await onConsoleLogsComplete;
+
+  // messages are coming from different targets so the order isn't guaranteed
+  const topLevelMessageResource = messages.find(resource =>
+    resource.message.filename.startsWith(URL_ROOT_COM_SSL)
+  );
+  const iframeMessage = messages.find(resource =>
+    resource.message.filename.startsWith("data:")
+  );
+
+  assertConsoleMessage(resourceCommand, topLevelMessageResource, {
+    targetFront: resourceCommand.targetCommand.targetFront,
+    messageText: "top-level document log",
+  });
+  assertConsoleMessage(resourceCommand, iframeMessage, {
+    targetFront: isEveryFrameTargetEnabled
+      ? resourceCommand.targetCommand
+          .getAllTargets([resourceCommand.targetCommand.TYPES.FRAME])
+          .find(t => t.url.startsWith("data:"))
+      : resourceCommand.targetCommand.targetFront,
+    messageText: "data url data log",
+  });
+
+  resourceCommand.unwatchResources([resourceCommand.TYPES.CONSOLE_MESSAGE], {
+    onAvailable,
+  });
+}
+
+async function testCrossProcessIframeNavigation(browser, resourceCommand) {
+  info("Navigate an inner iframe from data: URI to a https remote URL");
+
+  let doneResolve;
+  const messages = [];
+  const onConsoleLogsComplete = new Promise(resolve => (doneResolve = resolve));
+
+  const onAvailable = resources => {
+    messages.push(
+      ...resources.filter(
+        r => !r.message.arguments[0].startsWith("[WORKER] started")
+      )
+    );
+    if (messages.length == 3) {
+      doneResolve();
+    }
+  };
+
+  await resourceCommand.watchResources(
+    [resourceCommand.TYPES.CONSOLE_MESSAGE],
+    {
+      onAvailable,
+    }
+  );
+
+  // messages are coming from different targets so the order isn't guaranteed
+  const topLevelMessageResource = messages.find(resource =>
+    resource.message.arguments[0].startsWith("top-level")
+  );
+  const dataUrlMessageResource = messages.find(resource =>
+    resource.message.arguments[0].startsWith("data url")
+  );
+
+  // Assert cached messages from the previous top document
+  assertConsoleMessage(resourceCommand, topLevelMessageResource, {
+    messageText: "top-level document log",
+  });
+  assertConsoleMessage(resourceCommand, dataUrlMessageResource, {
+    messageText: "data url data log",
+  });
+
+  // Navigate the iframe to another origin/process
+  await SpecialPowers.spawn(browser, [TEST_IFRAME_URL], function (iframeUrl) {
+    const iframe = content.document.querySelector("iframe");
+    iframe.src = iframeUrl;
+  });
+
+  info("Wait for log message");
+  await onConsoleLogsComplete;
+
+  // iframeTarget will be different if Fission is on or off
+  const iframeTarget = await getIframeTargetFront(
+    resourceCommand.targetCommand
+  );
+
+  const iframeMessageResource = messages.find(resource =>
+    resource.message.arguments[0].endsWith("iframe log")
+  );
+  assertConsoleMessage(resourceCommand, iframeMessageResource, {
+    messageText: `${TEST_DOMAIN} iframe log`,
+    targetFront: iframeTarget,
+  });
+
+  resourceCommand.unwatchResources([resourceCommand.TYPES.CONSOLE_MESSAGE], {
+    onAvailable,
+  });
+}
+
+function assertConsoleMessage(resourceCommand, messageResource, expected) {
+  is(
+    messageResource.resourceType,
+    resourceCommand.TYPES.CONSOLE_MESSAGE,
+    "Resource is a console message"
+  );
+  ok(messageResource.message, "message is wrapped into a message attribute");
+  if (expected.targetFront) {
+    is(
+      messageResource.targetFront,
+      expected.targetFront,
+      "Message has the correct target front"
+    );
+  }
+  is(
+    messageResource.message.arguments[0],
+    expected.messageText,
+    "The correct type of message"
+  );
+}
+
+async function getIframeTargetFront(targetCommand) {
+  // If Fission/EFT is enabled, the iframe will have a dedicated target,
+  // otherwise it will be debuggable via the top level target.
+  if (!isFissionEnabled() && !isEveryFrameTargetEnabled()) {
+    return targetCommand.targetFront;
+  }
+  const frameTargets = targetCommand.getAllTargets([targetCommand.TYPES.FRAME]);
+  const browsingContextID = await SpecialPowers.spawn(
+    gBrowser.selectedBrowser,
+    [],
+    () => {
+      return content.document.querySelector("iframe").browsingContext.id;
+    }
+  );
+  const iframeTarget = frameTargets.find(target => {
+    return target.browsingContextID == browsingContextID;
+  });
+  ok(iframeTarget, "Found the iframe target front");
+  return iframeTarget;
+}
diff --git a/devtools/shared/commands/resource/tests/browser_resources_console_messages_workers.js b/devtools/shared/commands/resource/tests/browser_resources_console_messages_workers.js
new file mode 100644
index 0000000000..4b10f1d2e4
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/browser_resources_console_messages_workers.js
@@ -0,0 +1,257 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test the ResourceCommand API around CONSOLE_MESSAGE in workers
+
+const FISSION_TEST_URL = URL_ROOT_SSL + "fission_document_workers.html";
+const WORKER_FILE = "test_worker.js";
+const IFRAME_FILE = `${URL_ROOT_ORG_SSL}fission_iframe_workers.html`;
+
+add_task(async function () {
+  // Set the following pref to false as it's the one that enables direct connection
+  // to the worker targets
+  await pushPref("dom.worker.console.dispatch_events_to_main_thread", false);
+
+  const tab = await addTab(FISSION_TEST_URL);
+
+  const { client, resourceCommand, targetCommand } = await initResourceCommand(
+    tab,
+    { listenForWorkers: true }
+  );
+
+  info("Wait for the workers (from the main page and the iframe) to be ready");
+  const targets = [];
+  await new Promise(resolve => {
+    const onAvailable = async ({ targetFront }) => {
+      targets.push(targetFront);
+      if (targets.length === 2) {
+        resolve();
+      }
+    };
+    targetCommand.watchTargets({
+      types: [targetCommand.TYPES.WORKER],
+      onAvailable,
+    });
+  });
+
+  // The worker logs a message right when it starts, containing its location, so we can
+  // assert that we get the logs from the worker spawned in the content page and from the
+  // worker spawned in the iframe.
+  info("Check that we receive the cached messages");
+
+  const resources = [];
+  const onAvailable = innerResources => {
+    for (const resource of innerResources) {
+      // Ignore resources from non worker targets
+      if (!resource.targetFront.isWorkerTarget) {
+        continue;
+      }
+
+      resources.push(resource);
+    }
+  };
+
+  await resourceCommand.watchResources(
+    [resourceCommand.TYPES.CONSOLE_MESSAGE],
+    {
+      onAvailable,
+    }
+  );
+
+  is(resources.length, 2, "Got the expected number of existing messages");
+  const startLogFromWorkerInMainPage = resources.find(
+    ({ message }) =>
+      message.arguments[1] === `${URL_ROOT_SSL}${WORKER_FILE}#simple-worker`
+  );
+  const startLogFromWorkerInIframe = resources.find(
+    ({ message }) =>
+      message.arguments[1] ===
+      `${URL_ROOT_ORG_SSL}${WORKER_FILE}#simple-worker-in-iframe`
+  );
+
+  checkStartWorkerLogMessage(startLogFromWorkerInMainPage, {
+    expectedUrl: `${URL_ROOT_SSL}${WORKER_FILE}#simple-worker`,
+    isAlreadyExistingResource: true,
+  });
+  checkStartWorkerLogMessage(startLogFromWorkerInIframe, {
+    expectedUrl: `${URL_ROOT_ORG_SSL}${WORKER_FILE}#simple-worker-in-iframe`,
+    isAlreadyExistingResource: true,
+  });
+  let messageCount = resources.length;
+
+  info(
+    "Now log messages *after* the call to ResourceCommand.watchResources and after having received all existing messages"
+  );
+
+  await SpecialPowers.spawn(tab.linkedBrowser, [], () => {
+    content.wrappedJSObject.logMessageInWorker("live message from main page");
+
+    const iframe = content.document.querySelector("iframe");
+    SpecialPowers.spawn(iframe, [], () => {
+      content.wrappedJSObject.logMessageInWorker("live message from iframe");
+    });
+  });
+
+  // Wait until the 2 new logs are available
+  await waitUntil(() => resources.length === messageCount + 2);
+  const liveMessageFromWorkerInMainPage = resources.find(
+    ({ message }) => message.arguments[1] === "live message from main page"
+  );
+  const liveMessageFromWorkerInIframe = resources.find(
+    ({ message }) => message.arguments[1] === "live message from iframe"
+  );
+
+  checkLogInWorkerMessage(
+    liveMessageFromWorkerInMainPage,
+    "live message from main page"
+  );
+
+  checkLogInWorkerMessage(
+    liveMessageFromWorkerInIframe,
+    "live message from iframe"
+  );
+
+  // update the current number of resources received
+  messageCount = resources.length;
+
+  info("Now spawn new workers and log messages in main page and iframe");
+  await SpecialPowers.spawn(
+    tab.linkedBrowser,
+    [WORKER_FILE],
+    async workerUrl => {
+      const spawnedWorker = new content.Worker(`${workerUrl}#spawned-worker`);
+      spawnedWorker.postMessage({
+        type: "log-in-worker",
+        message: "live message in spawned worker from main page",
+      });
+
+      const iframe = content.document.querySelector("iframe");
+      SpecialPowers.spawn(iframe, [workerUrl], async innerWorkerUrl => {
+        const spawnedWorkerInIframe = new content.Worker(
+          `${innerWorkerUrl}#spawned-worker-in-iframe`
+        );
+        spawnedWorkerInIframe.postMessage({
+          type: "log-in-worker",
+          message: "live message in spawned worker from iframe",
+        });
+      });
+    }
+  );
+
+  info(
+    "Wait until the 4 new logs are available (the ones logged at worker creation + the ones from postMessage"
+  );
+  await waitUntil(
+    () => resources.length === messageCount + 4,
+    `Couldn't get the expected number of resources (expected ${
+      messageCount + 4
+    }, got ${resources.length})`
+  );
+  const startLogFromSpawnedWorkerInMainPage = resources.find(
+    ({ message }) =>
+      message.arguments[1] === `${URL_ROOT_SSL}${WORKER_FILE}#spawned-worker`
+  );
+  const startLogFromSpawnedWorkerInIframe = resources.find(
+    ({ message }) =>
+      message.arguments[1] ===
+      `${URL_ROOT_ORG_SSL}${WORKER_FILE}#spawned-worker-in-iframe`
+  );
+  const liveMessageFromSpawnedWorkerInMainPage = resources.find(
+    ({ message }) =>
+      message.arguments[1] === "live message in spawned worker from main page"
+  );
+  const liveMessageFromSpawnedWorkerInIframe = resources.find(
+    ({ message }) =>
+      message.arguments[1] === "live message in spawned worker from iframe"
+  );
+
+  checkStartWorkerLogMessage(startLogFromSpawnedWorkerInMainPage, {
+    expectedUrl: `${URL_ROOT_SSL}${WORKER_FILE}#spawned-worker`,
+  });
+  checkStartWorkerLogMessage(startLogFromSpawnedWorkerInIframe, {
+    expectedUrl: `${URL_ROOT_ORG_SSL}${WORKER_FILE}#spawned-worker-in-iframe`,
+  });
+  checkLogInWorkerMessage(
+    liveMessageFromSpawnedWorkerInMainPage,
+    "live message in spawned worker from main page"
+  );
+  checkLogInWorkerMessage(
+    liveMessageFromSpawnedWorkerInIframe,
+    "live message in spawned worker from iframe"
+  );
+  // update the current number of resources received
+  messageCount = resources.length;
+
+  info(
+    "Add a remote iframe on the same origin we already have an iframe and check we get the messages"
+  );
+  await SpecialPowers.spawn(
+    tab.linkedBrowser,
+    [IFRAME_FILE],
+    async iframeUrl => {
+      const iframe = content.document.createElement("iframe");
+      iframe.src = `${iframeUrl}?hashSuffix=in-second-iframe`;
+      content.document.body.append(iframe);
+    }
+  );
+
+  info("Wait until the new log is available");
+  await waitUntil(
+    () => resources.length === messageCount + 1,
+    `Couldn't get the expected number of resources (expected ${
+      messageCount + 1
+    }, got ${resources.length})`
+  );
+  const startLogFromWorkerInSecondIframe = resources[resources.length - 1];
+  checkStartWorkerLogMessage(startLogFromWorkerInSecondIframe, {
+    expectedUrl: `${URL_ROOT_ORG_SSL}${WORKER_FILE}#simple-worker-in-second-iframe`,
+  });
+
+  targetCommand.destroy();
+  await client.close();
+
+  await SpecialPowers.spawn(tab.linkedBrowser, [], async () => {
+    // registrationPromise is set by the test page.
+    const registration = await content.wrappedJSObject.registrationPromise;
+    registration.unregister();
+  });
+});
+
+function checkStartWorkerLogMessage(
+  resource,
+  { expectedUrl, isAlreadyExistingResource = false }
+) {
+  const { message } = resource;
+  const [firstArg, secondArg, thirdArg] = message.arguments;
+  is(firstArg, "[WORKER] started", "Got the expected first argument");
+  is(secondArg, expectedUrl, "expected url was logged");
+  is(
+    thirdArg?._grip?.class,
+    "DedicatedWorkerGlobalScope",
+    "the global scope was logged as expected"
+  );
+  is(
+    resource.isAlreadyExistingResource,
+    isAlreadyExistingResource,
+    "Resource has expected value for isAlreadyExistingResource"
+  );
+}
+
+function checkLogInWorkerMessage(resource, expectedMessage) {
+  const { message } = resource;
+  const [firstArg, secondArg, thirdArg] = message.arguments;
+  is(firstArg, "[WORKER]", "Got the expected first argument");
+  is(secondArg, expectedMessage, "expected message was logged");
+  is(
+    thirdArg?._grip?.class,
+    "MessageEvent",
+    "the message event object was logged as expected"
+  );
+  is(
+    resource.isAlreadyExistingResource,
+    false,
+    "Resource has expected value for isAlreadyExistingResource"
+  );
+}
diff --git a/devtools/shared/commands/resource/tests/browser_resources_css_changes.js b/devtools/shared/commands/resource/tests/browser_resources_css_changes.js
new file mode 100644
index 0000000000..22b11a8186
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/browser_resources_css_changes.js
@@ -0,0 +1,151 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test the ResourceCommand API around CSS_CHANGE.
+
+add_task(async function () {
+  // Open a test tab
+  const tab = await addTab(
+    "data:text/html,<body style='color: lime;'>CSS Changes</body>"
+  );
+
+  const { client, resourceCommand, targetCommand } = await initResourceCommand(
+    tab
+  );
+
+  // CSS_CHANGE watcher doesn't record modification made before watching,
+  // so we have to start watching before doing any DOM mutation.
+  await resourceCommand.watchResources([resourceCommand.TYPES.CSS_CHANGE], {
+    onAvailable: () => {},
+  });
+
+  const { walker } = await targetCommand.targetFront.getFront("inspector");
+  const nodeList = await walker.querySelectorAll(walker.rootNode, "body");
+  const body = (await nodeList.items())[0];
+  const style = (
+    await body.inspectorFront.pageStyle.getApplied(body, {
+      skipPseudo: false,
+    })
+  )[0];
+
+  info(
+    "Check whether ResourceCommand catches CSS change that fired before starting to watch"
+  );
+  await setProperty(style.rule, 0, "color", "black");
+
+  const availableResources = [];
+  await resourceCommand.watchResources([resourceCommand.TYPES.CSS_CHANGE], {
+    onAvailable: resources => availableResources.push(...resources),
+  });
+  assertResource(
+    availableResources[0],
+    { index: 0, property: "color", value: "black" },
+    { index: 0, property: "color", value: "lime" }
+  );
+
+  info(
+    "Check whether ResourceCommand catches CSS changes after the property was renamed and updated"
+  );
+
+  // RuleRewriter:apply will not support a simultaneous rename + setProperty.
+  // Doing so would send inconsistent arguments to StyleRuleActor:setRuleText,
+  // the CSS text for the rule will not match the list of modifications, which
+  // would desynchronize the Changes view. Thankfully this scenario should not
+  // happen when using the UI to update the rules.
+  await renameProperty(style.rule, 0, "color", "background-color");
+  await waitUntil(() => availableResources.length === 2);
+  assertResource(
+    availableResources[1],
+    { index: 0, property: "background-color", value: "black" },
+    { index: 0, property: "color", value: "black" }
+  );
+
+  await setProperty(style.rule, 0, "background-color", "pink");
+  await waitUntil(() => availableResources.length === 3);
+  assertResource(
+    availableResources[2],
+    { index: 0, property: "background-color", value: "pink" },
+    { index: 0, property: "background-color", value: "black" }
+  );
+
+  info("Check whether ResourceCommand catches CSS change of disabling");
+  await setPropertyEnabled(style.rule, 0, "background-color", false);
+  await waitUntil(() => availableResources.length === 4);
+  assertResource(availableResources[3], null, {
+    index: 0,
+    property: "background-color",
+    value: "pink",
+  });
+
+  info("Check whether ResourceCommand catches CSS change of new property");
+  await createProperty(style.rule, 1, "font-size", "100px");
+  await waitUntil(() => availableResources.length === 5);
+  assertResource(
+    availableResources[4],
+    { index: 1, property: "font-size", value: "100px" },
+    null
+  );
+
+  info("Check whether ResourceCommand sends all resources added in this test");
+  const existingResources = [];
+  await resourceCommand.watchResources([resourceCommand.TYPES.CSS_CHANGE], {
+    onAvailable: resources => existingResources.push(...resources),
+  });
+  await waitUntil(() => existingResources.length === 5);
+  is(availableResources[0], existingResources[0], "1st resource is correct");
+  is(availableResources[1], existingResources[1], "2nd resource is correct");
+  is(availableResources[2], existingResources[2], "3rd resource is correct");
+  is(availableResources[3], existingResources[3], "4th resource is correct");
+  is(availableResources[4], existingResources[4], "4th resource is correct");
+
+  targetCommand.destroy();
+  await client.close();
+});
+
+function assertResource(resource, expectedAddedChange, expectedRemovedChange) {
+  if (expectedAddedChange) {
+    is(resource.add.length, 1, "The number of added changes is correct");
+    assertChange(resource.add[0], expectedAddedChange);
+  } else {
+    is(resource.add, null, "There is no added changes");
+  }
+
+  if (expectedRemovedChange) {
+    is(resource.remove.length, 1, "The number of removed changes is correct");
+    assertChange(resource.remove[0], expectedRemovedChange);
+  } else {
+    is(resource.remove, null, "There is no removed changes");
+  }
+}
+
+function assertChange(change, expected) {
+  is(change.index, expected.index, "The index of change is correct");
+  is(change.property, expected.property, "The property of change is correct");
+  is(change.value, expected.value, "The value of change is correct");
+}
+
+async function setProperty(rule, index, property, value) {
+  const modifications = rule.startModifyingProperties({ isKnown: true });
+  modifications.setProperty(index, property, value, "");
+  await modifications.apply();
+}
+
+async function renameProperty(rule, index, oldName, newName, value) {
+  const modifications = rule.startModifyingProperties({ isKnown: true });
+  modifications.renameProperty(index, oldName, newName);
+  await modifications.apply();
+}
+
+async function createProperty(rule, index, property, value) {
+  const modifications = rule.startModifyingProperties({ isKnown: true });
+  modifications.createProperty(index, property, value, "", true);
+  await modifications.apply();
+}
+
+async function setPropertyEnabled(rule, index, property, isEnabled) {
+  const modifications = rule.startModifyingProperties({ isKnown: true });
+  modifications.setPropertyEnabled(index, property, isEnabled);
+  await modifications.apply();
+}
diff --git a/devtools/shared/commands/resource/tests/browser_resources_css_messages.js b/devtools/shared/commands/resource/tests/browser_resources_css_messages.js
new file mode 100644
index 0000000000..1b4b56cd4f
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/browser_resources_css_messages.js
@@ -0,0 +1,212 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test the ResourceCommand API around CSS_MESSAGE
+// Reproduces the CSS message assertions from devtools/shared/webconsole/test/chrome/test_page_errors.html
+
+const { MESSAGE_CATEGORY } = require("resource://devtools/shared/constants.js");
+
+// Create a simple server so we have a nice sourceName in the resources packets.
+const httpServer = createTestHTTPServer();
+httpServer.registerPathHandler(`/test_css_messages.html`, (req, res) => {
+  res.setStatusLine(req.httpVersion, 200, "OK");
+  res.write(`<meta charset=utf8>
+    <style>
+      html {
+        body {
+          color: bloup;
+        }
+      }
+    </style>Test CSS Messages`);
+});
+
+const TEST_URI = `http://localhost:${httpServer.identity.primaryPort}/test_css_messages.html`;
+
+add_task(async function () {
+  await testWatchingCssMessages();
+  await testWatchingCachedCssMessages();
+});
+
+async function testWatchingCssMessages() {
+  // Disable the preloaded process as it creates processes intermittently
+  // which forces the emission of RDP requests we aren't correctly waiting for.
+  await pushPref("dom.ipc.processPrelaunch.enabled", false);
+
+  // Open a test tab
+  const tab = await addTab(TEST_URI);
+
+  const { client, resourceCommand, targetCommand } = await initResourceCommand(
+    tab
+  );
+
+  const receivedMessages = [];
+  const { onAvailable, onAllMessagesReceived } = setupOnAvailableFunction(
+    targetCommand,
+    receivedMessages,
+    false
+  );
+  await resourceCommand.watchResources([resourceCommand.TYPES.CSS_MESSAGE], {
+    onAvailable,
+  });
+
+  info(
+    "Now log CSS warning *after* the call to ResourceCommand.watchResources and after " +
+      "having received the existing message"
+  );
+  // We need to wait for the first CSS Warning as it is not a cached message; when we
+  // start watching, the `cssErrorReportingEnabled` is checked on the target docShell, and
+  // if it is false, we re-parse the stylesheets to get the messages.
+  await BrowserTestUtils.waitForCondition(() => receivedMessages.length === 1);
+
+  info("Trigger a CSS Warning");
+  triggerCSSWarning(tab);
+
+  info("Waiting for all expected CSS messages to be received");
+  await onAllMessagesReceived;
+  ok(true, "All the expected CSS messages were received");
+
+  Services.console.reset();
+  targetCommand.destroy();
+  await client.close();
+}
+
+async function testWatchingCachedCssMessages() {
+  // Disable the preloaded process as it creates processes intermittently
+  // which forces the emission of RDP requests we aren't correctly waiting for.
+  await pushPref("dom.ipc.processPrelaunch.enabled", false);
+
+  // Open a test tab
+  const tab = await addTab(TEST_URI);
+
+  // By default, the CSS Parser does not emit warnings at all, for performance matter.
+  // Since we actually want the Parser to emit those messages _before_ we start listening
+  // for CSS messages, we need to set the cssErrorReportingEnabled flag on the docShell.
+  await SpecialPowers.spawn(gBrowser.selectedBrowser, [], function () {
+    content.docShell.cssErrorReportingEnabled = true;
+  });
+
+  // Setting the docShell flag only indicates to the Parser that from now on, it should
+  // emit warnings. But it does not automatically emit warnings for the existing CSS
+  // errors in the stylesheets. So here we reload the tab, which will make the Parser
+  // parse the stylesheets again, this time emitting warnings.
+  await reloadBrowser();
+  // and trigger more CSS warnings
+  await triggerCSSWarning(tab);
+
+  // At this point, all messages should be in the ConsoleService cache, and we can begin
+  // to watch and check that we do retrieve those messages.
+  const { client, resourceCommand, targetCommand } = await initResourceCommand(
+    tab
+  );
+
+  const receivedMessages = [];
+  const { onAvailable } = setupOnAvailableFunction(
+    targetCommand,
+    receivedMessages,
+    true
+  );
+  await resourceCommand.watchResources([resourceCommand.TYPES.CSS_MESSAGE], {
+    onAvailable,
+  });
+  is(receivedMessages.length, 3, "Cached messages were retrieved as expected");
+
+  Services.console.reset();
+  targetCommand.destroy();
+  await client.close();
+}
+
+function setupOnAvailableFunction(
+  targetCommand,
+  receivedMessages,
+  isAlreadyExistingResource
+) {
+  // timeStamp are the result of a number in microsecond divided by 1000.
+  // so we can't expect a precise number of decimals, or even if there would
+  // be decimals at all.
+  const FRACTIONAL_NUMBER_REGEX = /^\d+(\.\d{1,3})?$/;
+
+  // The expected messages are the CSS warnings:
+  // - one for the rule in the style element
+  // - two for the JS modified style we're doing in the test.
+  const expectedMessages = [
+    {
+      pageError: {
+        errorMessage: /Expected color but found ‘bloup’/,
+        sourceName: /test_css_messages/,
+        category: MESSAGE_CATEGORY.CSS_PARSER,
+        timeStamp: FRACTIONAL_NUMBER_REGEX,
+        error: false,
+        warning: true,
+      },
+      cssSelectors: ":is(html) body",
+      isAlreadyExistingResource,
+    },
+    {
+      pageError: {
+        errorMessage: /Error in parsing value for ‘width’/,
+        sourceName: /test_css_messages/,
+        category: MESSAGE_CATEGORY.CSS_PARSER,
+        timeStamp: FRACTIONAL_NUMBER_REGEX,
+        error: false,
+        warning: true,
+      },
+      isAlreadyExistingResource,
+    },
+    {
+      pageError: {
+        errorMessage: /Error in parsing value for ‘height’/,
+        sourceName: /test_css_messages/,
+        category: MESSAGE_CATEGORY.CSS_PARSER,
+        timeStamp: FRACTIONAL_NUMBER_REGEX,
+        error: false,
+        warning: true,
+      },
+      isAlreadyExistingResource,
+    },
+  ];
+
+  let done;
+  const onAllMessagesReceived = new Promise(resolve => (done = resolve));
+  const onAvailable = resources => {
+    for (const resource of resources) {
+      const { pageError } = resource;
+
+      is(
+        resource.targetFront,
+        targetCommand.targetFront,
+        "The targetFront property is the expected one"
+      );
+
+      if (!pageError.sourceName.includes("test_css_messages")) {
+        info(`Ignore error from unknown source: "${pageError.sourceName}"`);
+        continue;
+      }
+
+      const index = receivedMessages.length;
+      receivedMessages.push(resource);
+
+      info(
+        `checking received css message #${index}: ${pageError.errorMessage}`
+      );
+      ok(pageError, "The resource has a pageError attribute");
+      checkObject(resource, expectedMessages[index]);
+
+      if (receivedMessages.length == expectedMessages.length) {
+        done();
+      }
+    }
+  };
+  return { onAvailable, onAllMessagesReceived };
+}
+
+/**
+ * Sets invalid values for width and height on the document's body style attribute.
+ */
+function triggerCSSWarning(tab) {
+  return ContentTask.spawn(tab.linkedBrowser, null, function frameScript() {
+    content.document.body.style.width = "red";
+    content.document.body.style.height = "blue";
+  });
+}
diff --git a/devtools/shared/commands/resource/tests/browser_resources_css_registered_properties.js b/devtools/shared/commands/resource/tests/browser_resources_css_registered_properties.js
new file mode 100644
index 0000000000..1429b55167
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/browser_resources_css_registered_properties.js
@@ -0,0 +1,384 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test the ResourceCommand API around CSS_REGISTERED_PROPERTIES.
+
+const ResourceCommand = require("resource://devtools/shared/commands/resource/resource-command.js");
+
+const IFRAME_URL = `https://example.org/document-builder.sjs?html=${encodeURIComponent(`
+  <style>
+    @property --css-a {
+      syntax: "<color>";
+      inherits: true;
+      initial-value: gold;
+    }
+  </style>
+  <script>
+    CSS.registerProperty({
+      name: "--js-a",
+      syntax: "<length>",
+      inherits: true,
+      initialValue: "20px"
+    });
+  </script>
+  <h1>iframe</h1>
+`)}`;
+
+const TEST_URL = `https://example.org/document-builder.sjs?html=
+  <head>
+    <style>
+      @property --css-a {
+        syntax: "*";
+        inherits: false;
+      }
+
+      @property --css-b {
+        syntax: "<color>";
+        inherits: true;
+        initial-value: tomato;
+      }
+    </style>
+    <script>
+      CSS.registerProperty({
+        name: "--js-a",
+        syntax: "*",
+        inherits: false,
+      });
+      CSS.registerProperty({
+        name: "--js-b",
+        syntax: "<length>",
+        inherits: true,
+        initialValue: "10px"
+      });
+    </script>
+  </head>
+  <h1>CSS_REGISTERED_PROPERTIES</h1>
+  <iframe src="${encodeURIComponent(IFRAME_URL)}"></iframe>`;
+
+add_task(async function () {
+  await pushPref("layout.css.properties-and-values.enabled", true);
+  const tab = await addTab(TEST_URL);
+
+  const { client, resourceCommand, targetCommand } = await initResourceCommand(
+    tab
+  );
+
+  // Wait for targets
+  await targetCommand.startListening();
+  const targets = [];
+  const onAvailable = ({ targetFront }) => targets.push(targetFront);
+  await targetCommand.watchTargets({
+    types: [targetCommand.TYPES.FRAME],
+    onAvailable,
+  });
+  await waitFor(() => targets.length === 2);
+  const [topLevelTarget, iframeTarget] = targets.sort((a, b) =>
+    a.isTopLevel ? -1 : 1
+  );
+
+  // Watching for new stylesheets shouldn't be
+  const stylesheets = [];
+  await resourceCommand.watchResources([resourceCommand.TYPES.STYLESHEET], {
+    onAvailable: resources => stylesheets.push(...resources),
+    ignoreExistingResources: true,
+  });
+
+  info("Check that we get existing registered properties");
+  const availableResources = [];
+  const updatedResources = [];
+  const destroyedResources = [];
+  await resourceCommand.watchResources(
+    [resourceCommand.TYPES.CSS_REGISTERED_PROPERTIES],
+    {
+      onAvailable: resources => availableResources.push(...resources),
+      onUpdated: resources => updatedResources.push(...resources),
+      onDestroyed: resources => destroyedResources.push(...resources),
+    }
+  );
+
+  is(
+    availableResources.length,
+    6,
+    "The 6 existing registered properties where retrieved"
+  );
+
+  // Sort resources so we get them alphabetically ordered by their name, with the ones for
+  // the top level target displayed first.
+  availableResources.sort((a, b) => {
+    if (a.targetFront !== b.targetFront) {
+      return a.targetFront.isTopLevel ? -1 : 1;
+    }
+    return a.name < b.name ? -1 : 1;
+  });
+
+  assertResource(availableResources[0], {
+    name: "--css-a",
+    syntax: "*",
+    inherits: false,
+    initialValue: null,
+    fromJS: false,
+    targetFront: topLevelTarget,
+  });
+  assertResource(availableResources[1], {
+    name: "--css-b",
+    syntax: "<color>",
+    inherits: true,
+    initialValue: "tomato",
+    fromJS: false,
+    targetFront: topLevelTarget,
+  });
+  assertResource(availableResources[2], {
+    name: "--js-a",
+    syntax: "*",
+    inherits: false,
+    initialValue: null,
+    fromJS: true,
+    targetFront: topLevelTarget,
+  });
+  assertResource(availableResources[3], {
+    name: "--js-b",
+    syntax: "<length>",
+    inherits: true,
+    initialValue: "10px",
+    fromJS: true,
+    targetFront: topLevelTarget,
+  });
+  assertResource(availableResources[4], {
+    name: "--css-a",
+    syntax: "<color>",
+    inherits: true,
+    initialValue: "gold",
+    fromJS: false,
+    targetFront: iframeTarget,
+  });
+  assertResource(availableResources[5], {
+    name: "--js-a",
+    syntax: "<length>",
+    inherits: true,
+    initialValue: "20px",
+    fromJS: true,
+    targetFront: iframeTarget,
+  });
+
+  info("Check that we didn't get notified about existing stylesheets");
+  // wait a bit so we'd have the time to be notified about stylesheet resources
+  await wait(500);
+  is(
+    stylesheets.length,
+    0,
+    "Watching for registered properties does not notify about existing stylesheets resources"
+  );
+
+  info("Check that we get properties from new stylesheets");
+  await SpecialPowers.spawn(gBrowser.selectedBrowser, [], () => {
+    const s = content.document.createElement("style");
+    s.textContent = `
+        @property --css-c {
+          syntax: "<custom-ident>";
+          inherits: true;
+          initial-value: custom;
+        }
+
+        @property --css-d {
+          syntax: "big | bigger";
+          inherits: true;
+          initial-value: big;
+        }
+    `;
+    content.document.head.append(s);
+  });
+
+  info("Wait for registered properties to be available");
+  await waitFor(() => availableResources.length === 8);
+  ok(true, "Got notified about 2 new registered properties");
+  assertResource(availableResources[6], {
+    name: "--css-c",
+    syntax: "<custom-ident>",
+    inherits: true,
+    initialValue: "custom",
+    fromJS: false,
+    targetFront: topLevelTarget,
+  });
+  assertResource(availableResources[7], {
+    name: "--css-d",
+    syntax: "big | bigger",
+    inherits: true,
+    initialValue: "big",
+    fromJS: false,
+    targetFront: topLevelTarget,
+  });
+
+  info("Wait to be notified about the new stylesheet");
+  await waitFor(() => stylesheets.length === 1);
+  ok(true, "we do get notified about stylesheets");
+
+  info(
+    "Check that we get notified about properties registered via CSS.registerProperty"
+  );
+  await SpecialPowers.spawn(tab.linkedBrowser, [], () => {
+    content.CSS.registerProperty({
+      name: "--js-c",
+      syntax: "*",
+      inherits: false,
+      initialValue: 42,
+    });
+    content.CSS.registerProperty({
+      name: "--js-d",
+      syntax: "<color>#",
+      inherits: true,
+      initialValue: "blue,cyan",
+    });
+  });
+
+  await waitFor(() => availableResources.length === 10);
+  ok(true, "Got notified about 2 new registered properties");
+  assertResource(availableResources[8], {
+    name: "--js-c",
+    syntax: "*",
+    inherits: false,
+    initialValue: "42",
+    fromJS: true,
+    targetFront: topLevelTarget,
+  });
+  assertResource(availableResources[9], {
+    name: "--js-d",
+    syntax: "<color>#",
+    inherits: true,
+    initialValue: "blue,cyan",
+    fromJS: true,
+    targetFront: topLevelTarget,
+  });
+
+  info(
+    "Check that we get notified about properties registered via CSS.registerProperty in iframe"
+  );
+  const iframeBrowsingContext = await SpecialPowers.spawn(
+    tab.linkedBrowser,
+    [],
+    () => content.document.querySelector("iframe").browsingContext
+  );
+
+  await SpecialPowers.spawn(iframeBrowsingContext, [], () => {
+    content.CSS.registerProperty({
+      name: "--js-iframe",
+      syntax: "<color>#",
+      inherits: true,
+      initialValue: "red,salmon",
+    });
+  });
+
+  await waitFor(() => availableResources.length === 11);
+  ok(true, "Got notified about 2 new registered properties");
+  assertResource(availableResources[10], {
+    name: "--js-iframe",
+    syntax: "<color>#",
+    inherits: true,
+    initialValue: "red,salmon",
+    fromJS: true,
+    targetFront: iframeTarget,
+  });
+
+  info(
+    "Check that we get notified about destroyed properties when removing stylesheet"
+  );
+  // sanity check
+  is(destroyedResources.length, 0, "No destroyed resources yet");
+  await SpecialPowers.spawn(gBrowser.selectedBrowser, [], () => {
+    content.document.querySelector("style").remove();
+  });
+  await waitFor(() => destroyedResources.length == 2);
+  ok(true, "We got notified about destroyed resources");
+  destroyedResources.sort((a, b) => a < b);
+  is(
+    destroyedResources[0].resourceType,
+    ResourceCommand.TYPES.CSS_REGISTERED_PROPERTIES,
+    "resource type is correct"
+  );
+  is(
+    destroyedResources[0].resourceId,
+    `${topLevelTarget.actorID}:css-registered-property:--css-a`,
+    "expected css property was destroyed"
+  );
+  is(
+    destroyedResources[1].resourceType,
+    ResourceCommand.TYPES.CSS_REGISTERED_PROPERTIES,
+    "resource type is correct"
+  );
+  is(
+    destroyedResources[1].resourceId,
+    `${topLevelTarget.actorID}:css-registered-property:--css-b`,
+    "expected css property was destroyed"
+  );
+
+  info(
+    "Check that we get notified about updated properties when modifying stylesheet"
+  );
+  is(updatedResources.length, 0, "No updated resources yet");
+  await SpecialPowers.spawn(gBrowser.selectedBrowser, [], () => {
+    content.document.querySelector("style").textContent = `
+     /* not updated */
+      @property --css-c {
+        syntax: "<custom-ident>";
+        inherits: true;
+        initial-value: custom;
+      }
+
+      @property --css-d {
+        syntax: "big | bigger";
+        inherits: true;
+        /* only change initial value (was big) */
+        initial-value: bigger;
+      }
+
+      /* add a new property */
+      @property --css-e {
+        syntax: "<color>";
+        inherits: false;
+        initial-value: green;
+      }
+    `;
+  });
+  await waitFor(() => updatedResources.length === 1);
+  ok(true, "One property was updated");
+  assertResource(updatedResources[0].resource, {
+    name: "--css-d",
+    syntax: "big | bigger",
+    inherits: true,
+    initialValue: "bigger",
+    fromJS: false,
+    targetFront: topLevelTarget,
+  });
+
+  await waitFor(() => availableResources.length === 12);
+  ok(true, "We got notified about the new property");
+  assertResource(availableResources.at(-1), {
+    name: "--css-e",
+    syntax: "<color>",
+    inherits: false,
+    initialValue: "green",
+    fromJS: false,
+    targetFront: topLevelTarget,
+  });
+
+  await client.close();
+});
+
+async function assertResource(resource, expected) {
+  is(
+    resource.resourceType,
+    ResourceCommand.TYPES.CSS_REGISTERED_PROPERTIES,
+    "Resource type is correct"
+  );
+  is(resource.name, expected.name, "name is correct");
+  is(resource.syntax, expected.syntax, "syntax is correct");
+  is(resource.inherits, expected.inherits, "inherits is correct");
+  is(resource.initialValue, expected.initialValue, "initialValue is correct");
+  is(resource.fromJS, expected.fromJS, "fromJS is correct");
+  is(
+    resource.targetFront,
+    expected.targetFront,
+    "resource is associated with expected target"
+  );
+}
diff --git a/devtools/shared/commands/resource/tests/browser_resources_document_events.js b/devtools/shared/commands/resource/tests/browser_resources_document_events.js
new file mode 100644
index 0000000000..4692cba1ed
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/browser_resources_document_events.js
@@ -0,0 +1,720 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test the ResourceCommand API around DOCUMENT_EVENT
+
+add_task(async function () {
+  await testDocumentEventResources();
+  await testDocumentEventResourcesWithIgnoreExistingResources();
+  await testDomCompleteWithOverloadedConsole();
+  await testIframeNavigation();
+  await testBfCacheNavigation();
+  await testDomCompleteWithWindowStop();
+  await testCrossOriginNavigation();
+});
+
+async function testDocumentEventResources() {
+  info("Test ResourceCommand for DOCUMENT_EVENT");
+
+  // Open a test tab
+  const title = "DocumentEventsTitle";
+  const url = `data:text/html,<title>${title}</title>Document Events`;
+  const tab = await addTab(url);
+
+  const listener = new ResourceListener();
+  const { commands } = await initResourceCommand(tab);
+
+  info(
+    "Check whether the document events are fired correctly even when the document was already loaded"
+  );
+  const onLoadingAtInit = listener.once("dom-loading");
+  const onInteractiveAtInit = listener.once("dom-interactive");
+  const onCompleteAtInit = listener.once("dom-complete");
+  await commands.resourceCommand.watchResources(
+    [commands.resourceCommand.TYPES.DOCUMENT_EVENT],
+    {
+      onAvailable: parameters => listener.dispatch(parameters),
+    }
+  );
+  await assertPromises(
+    commands,
+    // targetBeforeNavigation is only used when there is a will-navigate and a navigate, but there is none here
+    null,
+    // As we started watching on an already loaded document, and no navigation happened since we called watchResources,
+    // we don't have any will-navigate event
+    null,
+    onLoadingAtInit,
+    onInteractiveAtInit,
+    onCompleteAtInit
+  );
+  ok(
+    true,
+    "Document events are fired even when the document was already loaded"
+  );
+  let domLoadingResource = await onLoadingAtInit;
+
+  is(
+    domLoadingResource.url,
+    url,
+    `resource ${domLoadingResource.name} has expected url`
+  );
+  is(
+    domLoadingResource.title,
+    undefined,
+    `resource ${domLoadingResource.name} does not have a title property`
+  );
+
+  let domInteractiveResource = await onInteractiveAtInit;
+  is(
+    domInteractiveResource.url,
+    url,
+    `resource ${domInteractiveResource.name} has expected url`
+  );
+  is(
+    domInteractiveResource.title,
+    title,
+    `resource ${domInteractiveResource.name} has expected title`
+  );
+  let domCompleteResource = await onCompleteAtInit;
+  is(
+    domCompleteResource.url,
+    undefined,
+    `resource ${domCompleteResource.name} does not have a url property`
+  );
+  is(
+    domCompleteResource.title,
+    undefined,
+    `resource ${domCompleteResource.name} does not have a title property`
+  );
+
+  info("Check whether the document events are fired correctly when reloading");
+  const onWillNavigate = listener.once("will-navigate");
+  const onLoadingAtReloaded = listener.once("dom-loading");
+  const onInteractiveAtReloaded = listener.once("dom-interactive");
+  const onCompleteAtReloaded = listener.once("dom-complete");
+  const targetBeforeNavigation = commands.targetCommand.targetFront;
+  gBrowser.reloadTab(tab);
+  await assertPromises(
+    commands,
+    targetBeforeNavigation,
+    onWillNavigate,
+    onLoadingAtReloaded,
+    onInteractiveAtReloaded,
+    onCompleteAtReloaded
+  );
+  ok(true, "Document events are fired after reloading");
+
+  domLoadingResource = await onLoadingAtReloaded;
+  is(
+    domLoadingResource.url,
+    url,
+    `resource ${domLoadingResource.name} has expected url after reloading`
+  );
+  is(
+    domLoadingResource.title,
+    undefined,
+    `resource ${domLoadingResource.name} does not have a title property after reloading`
+  );
+
+  domInteractiveResource = await onInteractiveAtInit;
+  is(
+    domInteractiveResource.url,
+    url,
+    `resource ${domInteractiveResource.name} has url property after reloading`
+  );
+  is(
+    domInteractiveResource.title,
+    title,
+    `resource ${domInteractiveResource.name} has expected title after reloading`
+  );
+  domCompleteResource = await onCompleteAtInit;
+  is(
+    domCompleteResource.url,
+    undefined,
+    `resource ${domCompleteResource.name} does not have a url property after reloading`
+  );
+  is(
+    domCompleteResource.title,
+    undefined,
+    `resource ${domCompleteResource.name} does not have a title property after reloading`
+  );
+
+  await commands.destroy();
+}
+
+async function testDocumentEventResourcesWithIgnoreExistingResources() {
+  info("Test ignoreExistingResources option for DOCUMENT_EVENT");
+
+  const tab = await addTab("data:text/html,Document Events");
+
+  const { commands } = await initResourceCommand(tab);
+
+  info("Check whether the existing document events will not be fired");
+  const documentEvents = [];
+  await commands.resourceCommand.watchResources(
+    [commands.resourceCommand.TYPES.DOCUMENT_EVENT],
+    {
+      onAvailable: resources => documentEvents.push(...resources),
+      ignoreExistingResources: true,
+    }
+  );
+  is(documentEvents.length, 0, "Existing document events are not fired");
+
+  info("Check whether the future document events are fired");
+  const targetBeforeNavigation = commands.targetCommand.targetFront;
+  gBrowser.reloadTab(tab);
+  info(
+    "Wait for will-navigate, dom-loading, dom-interactive and dom-complete events"
+  );
+  await waitFor(() => documentEvents.length === 4);
+  assertEvents({ commands, targetBeforeNavigation, documentEvents });
+
+  await commands.destroy();
+}
+
+async function testIframeNavigation() {
+  info("Test iframe navigations for DOCUMENT_EVENT");
+
+  const tab = await addTab(
+    'https://example.com/document-builder.sjs?html=<iframe src="https://example.net/document-builder.sjs?html=net"></iframe>'
+  );
+  const secondPageUrl = "https://example.org/document-builder.sjs?html=org";
+
+  const { commands } = await initResourceCommand(tab);
+
+  let documentEvents = [];
+  await commands.resourceCommand.watchResources(
+    [commands.resourceCommand.TYPES.DOCUMENT_EVENT],
+    {
+      onAvailable: resources => documentEvents.push(...resources),
+    }
+  );
+  let iframeTarget;
+  if (isFissionEnabled() || isEveryFrameTargetEnabled()) {
+    is(
+      documentEvents.length,
+      6,
+      "With fission/EFT, we get two targets and two sets of events: dom-loading, dom-interactive, dom-complete"
+    );
+    [, iframeTarget] = await commands.targetCommand.getAllTargets([
+      commands.targetCommand.TYPES.FRAME,
+    ]);
+    // Filter out each target events as their order to be random between the two targets
+    const topTargetEvents = documentEvents.filter(
+      r => r.targetFront == commands.targetCommand.targetFront
+    );
+    const iframeTargetEvents = documentEvents.filter(
+      r => r.targetFront != commands.targetCommand.targetFront
+    );
+    assertEvents({
+      commands,
+      documentEvents: [null /* no will-navigate */, ...topTargetEvents],
+    });
+    assertEvents({
+      commands,
+      documentEvents: [null /* no will-navigate */, ...iframeTargetEvents],
+      expectedTargetFront: iframeTarget,
+    });
+  } else {
+    assertEvents({
+      commands,
+      documentEvents: [null /* no will-navigate */, ...documentEvents],
+    });
+  }
+
+  info("Navigate the iframe to another process (if fission is enabled)");
+  documentEvents = [];
+  await SpecialPowers.spawn(
+    gBrowser.selectedBrowser,
+    [secondPageUrl],
+    function (url) {
+      const iframe = content.document.querySelector("iframe");
+      iframe.src = url;
+    }
+  );
+
+  // We are switching to a new target only when fission is enabled...
+  if (isFissionEnabled() || isEveryFrameTargetEnabled()) {
+    await waitFor(() => documentEvents.length >= 3);
+    is(
+      documentEvents.length,
+      3,
+      "With fission/EFT, we switch to a new target and get: dom-loading, dom-interactive, dom-complete (but no will-navigate as that's only for the top BrowsingContext)"
+    );
+    const [, newIframeTarget] = await commands.targetCommand.getAllTargets([
+      commands.targetCommand.TYPES.FRAME,
+    ]);
+    assertEvents({
+      commands,
+      targetBeforeNavigation: iframeTarget,
+      documentEvents: [null /* no will-navigate */, ...documentEvents],
+      expectedTargetFront: newIframeTarget,
+      expectedNewURI: secondPageUrl,
+    });
+  } else {
+    // Wait for some time in order to let a chance to receive some unexpected events
+    await wait(250);
+    is(
+      documentEvents.length,
+      0,
+      "If fission is disabled, we navigate within the same process, we get no new target and no new resource"
+    );
+  }
+
+  await commands.destroy();
+}
+
+function isBfCacheInParentEnabled() {
+  return (
+    Services.appinfo.sessionHistoryInParent &&
+    Services.prefs.getBoolPref("fission.bfcacheInParent", false)
+  );
+}
+
+async function testBfCacheNavigation() {
+  info("Test bfcache navigations for DOCUMENT_EVENT");
+
+  info("Open a first document and navigate to a second one");
+  const firstLocation = "data:text/html,<title>first</title>first page";
+  const secondLocation = "data:text/html,<title>second</title>second page";
+  const tab = await addTab(firstLocation);
+  const onLoaded = BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser);
+  BrowserTestUtils.startLoadingURIString(
+    gBrowser.selectedBrowser,
+    secondLocation
+  );
+  await onLoaded;
+
+  const { commands } = await initResourceCommand(tab);
+
+  const documentEvents = [];
+  await commands.resourceCommand.watchResources(
+    [commands.resourceCommand.TYPES.DOCUMENT_EVENT],
+    {
+      onAvailable: resources => {
+        documentEvents.push(...resources);
+      },
+      ignoreExistingResources: true,
+    }
+  );
+  // Wait for some time for extra safety
+  await wait(250);
+  is(documentEvents.length, 0, "Existing document events are not fired");
+
+  info("Navigate back to the first page");
+  const onSwitched = commands.targetCommand.once("switched-target");
+  const targetBeforeNavigation = commands.targetCommand.targetFront;
+  gBrowser.goBack();
+
+  // We are switching to a new target only when fission/EFT is enabled...
+  if (
+    (isFissionEnabled() || isEveryFrameTargetEnabled()) &&
+    isBfCacheInParentEnabled()
+  ) {
+    await onSwitched;
+  }
+
+  info(
+    "Wait for will-navigate, dom-loading, dom-interactive and dom-complete events"
+  );
+  await waitFor(() => documentEvents.length >= 4);
+  /* Ignore will-navigate timestamp as all other DOCUMENT_EVENTS will be set at the original load date,
+     which is when we loaded from the network, and not when we loaded from bfcache */
+  assertEvents({
+    commands,
+    targetBeforeNavigation,
+    documentEvents,
+    ignoreWillNavigateTimestamp: true,
+  });
+
+  // Wait for some time in order to let a chance to have duplicated dom-loading events
+  await wait(250);
+
+  is(
+    documentEvents.length,
+    4,
+    "There is no duplicated event and only the 4 expected DOCUMENT_EVENT states"
+  );
+  const [willNavigateEvent, loadingEvent, interactiveEvent, completeEvent] =
+    documentEvents;
+
+  is(
+    willNavigateEvent.name,
+    "will-navigate",
+    "The first DOCUMENT_EVENT is will-navigate"
+  );
+  is(
+    loadingEvent.name,
+    "dom-loading",
+    "The second DOCUMENT_EVENT is dom-loading"
+  );
+  is(
+    interactiveEvent.name,
+    "dom-interactive",
+    "The third DOCUMENT_EVENT is dom-interactive"
+  );
+  is(
+    completeEvent.name,
+    "dom-complete",
+    "The fourth DOCUMENT_EVENT is dom-complete"
+  );
+
+  is(
+    loadingEvent.url,
+    firstLocation,
+    `resource ${loadingEvent.name} has expected url after navigation back`
+  );
+  is(
+    loadingEvent.title,
+    undefined,
+    `resource ${loadingEvent.name} does not have a title property after navigating back`
+  );
+
+  is(
+    interactiveEvent.url,
+    firstLocation,
+    `resource ${interactiveEvent.name} has expected url property after navigating back`
+  );
+  is(
+    interactiveEvent.title,
+    "first",
+    `resource ${interactiveEvent.name} has expected title after navigating back`
+  );
+
+  is(
+    completeEvent.url,
+    undefined,
+    `resource ${completeEvent.name} does not have a url property after navigating back`
+  );
+  is(
+    completeEvent.title,
+    undefined,
+    `resource ${completeEvent.name} does not have a title property after navigating back`
+  );
+
+  await commands.destroy();
+}
+
+async function testCrossOriginNavigation() {
+  info("Test cross origin navigations for DOCUMENT_EVENT");
+
+  const tab = await addTab("https://example.com/document-builder.sjs?html=com");
+
+  const { commands } = await initResourceCommand(tab);
+
+  const documentEvents = [];
+  await commands.resourceCommand.watchResources(
+    [commands.resourceCommand.TYPES.DOCUMENT_EVENT],
+    {
+      onAvailable: resources => documentEvents.push(...resources),
+      ignoreExistingResources: true,
+    }
+  );
+  // Wait for some time for extra safety
+  await wait(250);
+  is(documentEvents.length, 0, "Existing document events are not fired");
+
+  info("Navigate to another process");
+  const onSwitched = commands.targetCommand.once("switched-target");
+  const netUrl =
+    "https://example.net/document-builder.sjs?html=<head><title>titleNet</title></head>net";
+  const onLoaded = BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser);
+  const targetBeforeNavigation = commands.targetCommand.targetFront;
+  BrowserTestUtils.startLoadingURIString(gBrowser.selectedBrowser, netUrl);
+  await onLoaded;
+
+  // We are switching to a new target only when fission is enabled...
+  if (isFissionEnabled() || isEveryFrameTargetEnabled()) {
+    await onSwitched;
+  }
+
+  info(
+    "Wait for will-navigate, dom-loading, dom-interactive and dom-complete events"
+  );
+  await waitFor(() => documentEvents.length >= 4);
+  assertEvents({ commands, targetBeforeNavigation, documentEvents });
+
+  // Wait for some time in order to let a chance to have duplicated dom-loading events
+  await wait(250);
+
+  is(
+    documentEvents.length,
+    4,
+    "There is no duplicated event and only the 4 expected DOCUMENT_EVENT states"
+  );
+  const [willNavigateEvent, loadingEvent, interactiveEvent, completeEvent] =
+    documentEvents;
+
+  is(
+    willNavigateEvent.name,
+    "will-navigate",
+    "The first DOCUMENT_EVENT is will-navigate"
+  );
+  is(
+    loadingEvent.name,
+    "dom-loading",
+    "The second DOCUMENT_EVENT is dom-loading"
+  );
+  is(
+    interactiveEvent.name,
+    "dom-interactive",
+    "The third DOCUMENT_EVENT is dom-interactive"
+  );
+  is(
+    completeEvent.name,
+    "dom-complete",
+    "The fourth DOCUMENT_EVENT is dom-complete"
+  );
+
+  is(
+    loadingEvent.url,
+    encodeURI(netUrl),
+    `resource ${loadingEvent.name} has expected url after reloading`
+  );
+  is(
+    loadingEvent.title,
+    undefined,
+    `resource ${loadingEvent.name} does not have a title property after reloading`
+  );
+
+  is(
+    interactiveEvent.url,
+    encodeURI(netUrl),
+    `resource ${interactiveEvent.name} has expected url property after reloading`
+  );
+  is(
+    interactiveEvent.title,
+    "titleNet",
+    `resource ${interactiveEvent.name} has expected title after reloading`
+  );
+
+  is(
+    completeEvent.url,
+    undefined,
+    `resource ${completeEvent.name} does not have a url property after reloading`
+  );
+  is(
+    completeEvent.title,
+    undefined,
+    `resource ${completeEvent.name} does not have a title property after reloading`
+  );
+
+  await commands.destroy();
+}
+
+async function testDomCompleteWithOverloadedConsole() {
+  info("Test dom-complete with an overloaded console object");
+
+  const tab = await addTab(
+    "data:text/html,<script>window.console = {};</script>"
+  );
+
+  const { client, resourceCommand, targetCommand } = await initResourceCommand(
+    tab
+  );
+
+  info("Check that all DOCUMENT_EVENTS are fired for the already loaded page");
+  const documentEvents = [];
+  await resourceCommand.watchResources([resourceCommand.TYPES.DOCUMENT_EVENT], {
+    onAvailable: resources => documentEvents.push(...resources),
+  });
+  is(documentEvents.length, 3, "Existing document events are fired");
+
+  const domComplete = documentEvents[2];
+  is(domComplete.name, "dom-complete", "the last resource is the dom-complete");
+  is(
+    domComplete.hasNativeConsoleAPI,
+    false,
+    "the console object is reported to be overloaded"
+  );
+
+  targetCommand.destroy();
+  await client.close();
+}
+
+async function testDomCompleteWithWindowStop() {
+  info("Test dom-complete with a page calling window.stop()");
+
+  const tab = await addTab("data:text/html,foo");
+
+  const { commands, client, resourceCommand, targetCommand } =
+    await initResourceCommand(tab);
+
+  info("Check that all DOCUMENT_EVENTS are fired for the already loaded page");
+  let documentEvents = [];
+  await resourceCommand.watchResources([resourceCommand.TYPES.DOCUMENT_EVENT], {
+    onAvailable: resources => documentEvents.push(...resources),
+  });
+  is(documentEvents.length, 3, "Existing document events are fired");
+  documentEvents = [];
+
+  const html = `<!DOCTYPE html><html>
+  <head>
+    <title>stopped page</title>
+    <script>window.stop();</script>
+  </head>
+  <body>Page content that shouldn't be displayed</body>
+</html>`;
+  const secondLocation = "data:text/html," + encodeURIComponent(html);
+  const targetBeforeNavigation = commands.targetCommand.targetFront;
+  BrowserTestUtils.startLoadingURIString(
+    gBrowser.selectedBrowser,
+    secondLocation
+  );
+  info(
+    "Wait for will-navigate, dom-loading, dom-interactive and dom-complete events"
+  );
+  await waitFor(() => documentEvents.length === 4);
+
+  assertEvents({ commands, targetBeforeNavigation, documentEvents });
+
+  targetCommand.destroy();
+  await client.close();
+}
+
+async function assertPromises(
+  commands,
+  targetBeforeNavigation,
+  onWillNavigate,
+  onLoading,
+  onInteractive,
+  onComplete
+) {
+  const willNavigateEvent = await onWillNavigate;
+  const loadingEvent = await onLoading;
+  const interactiveEvent = await onInteractive;
+  const completeEvent = await onComplete;
+  assertEvents({
+    commands,
+    targetBeforeNavigation,
+    documentEvents: [
+      willNavigateEvent,
+      loadingEvent,
+      interactiveEvent,
+      completeEvent,
+    ],
+  });
+}
+
+function assertEvents({
+  commands,
+  targetBeforeNavigation,
+  documentEvents,
+  expectedTargetFront = commands.targetCommand.targetFront,
+  expectedNewURI = gBrowser.selectedBrowser.currentURI.spec,
+  ignoreWillNavigateTimestamp = false,
+}) {
+  const [willNavigateEvent, loadingEvent, interactiveEvent, completeEvent] =
+    documentEvents;
+  if (willNavigateEvent) {
+    is(willNavigateEvent.name, "will-navigate", "Received the will-navigate");
+    is(
+      willNavigateEvent.newURI,
+      expectedNewURI,
+      "will-navigate newURI is set to the current tab new location"
+    );
+  }
+  is(
+    loadingEvent.name,
+    "dom-loading",
+    "loading received in the exepected order"
+  );
+  is(
+    interactiveEvent.name,
+    "dom-interactive",
+    "interactive received in the expected order"
+  );
+  is(completeEvent.name, "dom-complete", "complete received last");
+
+  if (willNavigateEvent) {
+    is(
+      typeof willNavigateEvent.time,
+      "number",
+      `Type of time attribute for will-navigate event is correct (${willNavigateEvent.time})`
+    );
+  }
+  is(
+    typeof loadingEvent.time,
+    "number",
+    `Type of time attribute for loading event is correct (${loadingEvent.time})`
+  );
+  is(
+    typeof interactiveEvent.time,
+    "number",
+    `Type of time attribute for interactive event is correct (${interactiveEvent.time})`
+  );
+  is(
+    typeof completeEvent.time,
+    "number",
+    `Type of time attribute for complete event is correct (${completeEvent.time})`
+  );
+
+  if (willNavigateEvent && !ignoreWillNavigateTimestamp) {
+    Assert.lessOrEqual(
+      willNavigateEvent.time,
+      loadingEvent.time,
+      `Timestamp for dom-loading event is greater than will-navigate event (${willNavigateEvent.time} <= ${loadingEvent.time})`
+    );
+  }
+  Assert.lessOrEqual(
+    loadingEvent.time,
+    interactiveEvent.time,
+    `Timestamp for interactive event is greater than loading event (${loadingEvent.time} <= ${interactiveEvent.time})`
+  );
+  Assert.lessOrEqual(
+    interactiveEvent.time,
+    completeEvent.time,
+    `Timestamp for complete event is greater than interactive event (${interactiveEvent.time} <= ${completeEvent.time}).`
+  );
+
+  if (willNavigateEvent) {
+    // If we switched to a new target, this target will be different from currentTargetFront.
+    // This only happen if we navigate to another process or if server target switching is enabled.
+    is(
+      willNavigateEvent.targetFront,
+      targetBeforeNavigation,
+      "will-navigate target was the one before the navigation"
+    );
+  }
+  is(
+    loadingEvent.targetFront,
+    expectedTargetFront,
+    "loading target is the expected one"
+  );
+  is(
+    interactiveEvent.targetFront,
+    expectedTargetFront,
+    "interactive target is the expected one"
+  );
+  is(
+    completeEvent.targetFront,
+    expectedTargetFront,
+    "complete target is the expected one"
+  );
+
+  is(
+    completeEvent.hasNativeConsoleAPI,
+    true,
+    "None of the tests (except the dedicated one) overload the console object"
+  );
+}
+
+class ResourceListener {
+  _listeners = new Map();
+
+  dispatch(resources) {
+    for (const resource of resources) {
+      const resolve = this._listeners.get(resource.name);
+      if (resolve) {
+        resolve(resource);
+        this._listeners.delete(resource.name);
+      }
+    }
+  }
+
+  once(resourceName) {
+    return new Promise(r => this._listeners.set(resourceName, r));
+  }
+}
diff --git a/devtools/shared/commands/resource/tests/browser_resources_error_messages.js b/devtools/shared/commands/resource/tests/browser_resources_error_messages.js
new file mode 100644
index 0000000000..6f94266e4c
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/browser_resources_error_messages.js
@@ -0,0 +1,877 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test the ResourceCommand API around ERROR_MESSAGE
+// Reproduces assertions from devtools/shared/webconsole/test/chrome/test_page_errors.html
+
+// Create a simple server so we have a nice sourceName in the resources packets.
+const httpServer = createTestHTTPServer();
+httpServer.registerPathHandler(`/test_page_errors.html`, (req, res) => {
+  res.setStatusLine(req.httpVersion, 200, "OK");
+  res.write(`<!DOCTYPE html><meta charset=utf8>Test Error Messages`);
+});
+
+const TEST_URI = `http://localhost:${httpServer.identity.primaryPort}/test_page_errors.html`;
+
+add_task(async function () {
+  // Disable the preloaded process as it creates processes intermittently
+  // which forces the emission of RDP requests we aren't correctly waiting for.
+  await pushPref("dom.ipc.processPrelaunch.enabled", false);
+
+  await testErrorMessagesResources();
+  await testErrorMessagesResourcesWithIgnoreExistingResources();
+});
+
+async function testErrorMessagesResources() {
+  // Open a test tab
+  const tab = await addTab(TEST_URI);
+
+  const { client, resourceCommand, targetCommand } = await initResourceCommand(
+    tab
+  );
+
+  const receivedMessages = [];
+  // The expected messages are the errors, twice (once for cached messages, once for live messages)
+  const expectedMessages = Array.from(expectedPageErrors.values()).concat(
+    Array.from(expectedPageErrors.values())
+  );
+
+  info(
+    "Log some errors *before* calling ResourceCommand.watchResources in order to assert" +
+      " the behavior of already existing messages."
+  );
+  await triggerErrors(tab);
+
+  let done;
+  const onAllErrorReceived = new Promise(resolve => (done = resolve));
+  const onAvailable = resources => {
+    for (const resource of resources) {
+      const { pageError } = resource;
+
+      is(
+        resource.targetFront,
+        targetCommand.targetFront,
+        "The targetFront property is the expected one"
+      );
+
+      if (!pageError.sourceName.includes("test_page_errors")) {
+        info(`Ignore error from unknown source: "${pageError.sourceName}"`);
+        continue;
+      }
+
+      const index = receivedMessages.length;
+      receivedMessages.push(resource);
+
+      const isAlreadyExistingResource =
+        receivedMessages.length <= expectedPageErrors.size;
+      is(
+        resource.isAlreadyExistingResource,
+        isAlreadyExistingResource,
+        "isAlreadyExistingResource has expected value"
+      );
+
+      info(`checking received page error #${index}: ${pageError.errorMessage}`);
+      ok(pageError, "The resource has a pageError attribute");
+      checkPageErrorResource(pageError, expectedMessages[index]);
+
+      if (receivedMessages.length == expectedMessages.length) {
+        done();
+      }
+    }
+  };
+
+  await resourceCommand.watchResources([resourceCommand.TYPES.ERROR_MESSAGE], {
+    onAvailable,
+  });
+
+  await BrowserTestUtils.waitForCondition(
+    () => receivedMessages.length === expectedPageErrors.size
+  );
+
+  info(
+    "Now log errors *after* the call to ResourceCommand.watchResources and after having" +
+      " received all existing messages"
+  );
+  await triggerErrors(tab);
+
+  info("Waiting for all expected errors to be received");
+  await onAllErrorReceived;
+  ok(true, "All the expected errors were received");
+
+  Services.console.reset();
+  targetCommand.destroy();
+  await client.close();
+}
+
+async function testErrorMessagesResourcesWithIgnoreExistingResources() {
+  info("Test ignoreExistingResources option for ERROR_MESSAGE");
+  const tab = await addTab(TEST_URI);
+
+  const { client, resourceCommand, targetCommand } = await initResourceCommand(
+    tab
+  );
+
+  info(
+    "Check whether onAvailable will not be called with existing error messages"
+  );
+  await triggerErrors(tab);
+
+  const availableResources = [];
+  await resourceCommand.watchResources([resourceCommand.TYPES.ERROR_MESSAGE], {
+    onAvailable: resources => availableResources.push(...resources),
+    ignoreExistingResources: true,
+  });
+  is(
+    availableResources.length,
+    0,
+    "onAvailable wasn't called for existing error messages"
+  );
+
+  info(
+    "Check whether onAvailable will be called with the future error messages"
+  );
+  await triggerErrors(tab);
+
+  const expectedMessages = Array.from(expectedPageErrors.values());
+  await waitUntil(() => availableResources.length === expectedMessages.length);
+  for (let i = 0; i < expectedMessages.length; i++) {
+    const resource = availableResources[i];
+    const { pageError } = resource;
+    const expected = expectedMessages[i];
+    checkPageErrorResource(pageError, expected);
+    is(
+      resource.isAlreadyExistingResource,
+      false,
+      "isAlreadyExistingResource is set to false for live messages"
+    );
+  }
+
+  Services.console.reset();
+  targetCommand.destroy();
+  await client.close();
+}
+
+/**
+ * Triggers all the errors in the content page.
+ */
+async function triggerErrors(tab) {
+  for (const [expression, expected] of expectedPageErrors.entries()) {
+    if (
+      !expected[noUncaughtException] &&
+      !Services.appinfo.browserTabsRemoteAutostart
+    ) {
+      expectUncaughtException();
+    }
+
+    await ContentTask.spawn(
+      tab.linkedBrowser,
+      expression,
+      function frameScript(expr) {
+        const document = content.document;
+        const scriptEl = document.createElement("script");
+        scriptEl.textContent = expr;
+        document.body.appendChild(scriptEl);
+      }
+    );
+
+    if (expected.isPromiseRejection) {
+      // Wait a bit after an uncaught promise rejection error, as they are not emitted
+      // right away.
+
+      // eslint-disable-next-line mozilla/no-arbitrary-setTimeout
+      await new Promise(res => setTimeout(res, 10));
+    }
+  }
+}
+
+function checkPageErrorResource(pageErrorResource, expected) {
+  // Let's remove test harness related frames in stacktrace
+  const clonedPageErrorResource = { ...pageErrorResource };
+  if (clonedPageErrorResource.stacktrace) {
+    const index = clonedPageErrorResource.stacktrace.findIndex(frame =>
+      frame.filename.startsWith("resource://testing-common/content-task.js")
+    );
+    if (index > -1) {
+      clonedPageErrorResource.stacktrace =
+        clonedPageErrorResource.stacktrace.slice(0, index);
+    }
+  }
+  checkObject(clonedPageErrorResource, expected);
+}
+
+const noUncaughtException = Symbol();
+const NUMBER_REGEX = /^\d+$/;
+// timeStamp are the result of a number in microsecond divided by 1000.
+// so we can't expect a precise number of decimals, or even if there would
+// be decimals at all.
+const FRACTIONAL_NUMBER_REGEX = /^\d+(\.\d{1,3})?$/;
+
+const mdnUrl = path =>
+  `https://developer.mozilla.org/${path}?utm_source=mozilla&utm_medium=firefox-console-errors&utm_campaign=default`;
+
+const expectedPageErrors = new Map([
+  [
+    "document.doTheImpossible();",
+    {
+      errorMessage: /doTheImpossible/,
+      errorMessageName: "JSMSG_NOT_FUNCTION",
+      sourceName: /test_page_errors/,
+      category: "content javascript",
+      timeStamp: FRACTIONAL_NUMBER_REGEX,
+      error: true,
+      warning: false,
+      info: false,
+      lineText: "",
+      lineNumber: NUMBER_REGEX,
+      columnNumber: NUMBER_REGEX,
+      exceptionDocURL: mdnUrl(
+        "docs/Web/JavaScript/Reference/Errors/Not_a_function"
+      ),
+      innerWindowID: NUMBER_REGEX,
+      private: false,
+      stacktrace: [
+        {
+          filename: /test_page_errors\.html/,
+          lineNumber: 1,
+          columnNumber: 10,
+          functionName: null,
+        },
+      ],
+      notes: null,
+      chromeContext: false,
+      isPromiseRejection: false,
+      isForwardedFromContentProcess: false,
+    },
+  ],
+  [
+    "(42).toString(0);",
+    {
+      errorMessage: /radix/,
+      errorMessageName: "JSMSG_BAD_RADIX",
+      sourceName: /test_page_errors/,
+      category: "content javascript",
+      timeStamp: FRACTIONAL_NUMBER_REGEX,
+      error: true,
+      warning: false,
+      info: false,
+      lineText: "",
+      lineNumber: NUMBER_REGEX,
+      columnNumber: NUMBER_REGEX,
+      exceptionDocURL: mdnUrl("docs/Web/JavaScript/Reference/Errors/Bad_radix"),
+      innerWindowID: NUMBER_REGEX,
+      private: false,
+      stacktrace: [
+        {
+          filename: /test_page_errors\.html/,
+          lineNumber: 1,
+          columnNumber: 6,
+          functionName: null,
+        },
+      ],
+      notes: null,
+      chromeContext: false,
+      isPromiseRejection: false,
+      isForwardedFromContentProcess: false,
+    },
+  ],
+  [
+    "'use strict'; (Object.freeze({name: 'Elsa', score: 157})).score = 0;",
+    {
+      errorMessage: /read.only/,
+      errorMessageName: "JSMSG_READ_ONLY",
+      sourceName: /test_page_errors/,
+      category: "content javascript",
+      timeStamp: FRACTIONAL_NUMBER_REGEX,
+      error: true,
+      warning: false,
+      info: false,
+      lineText: "",
+      lineNumber: NUMBER_REGEX,
+      columnNumber: NUMBER_REGEX,
+      exceptionDocURL: mdnUrl("docs/Web/JavaScript/Reference/Errors/Read-only"),
+      innerWindowID: NUMBER_REGEX,
+      private: false,
+      stacktrace: [
+        {
+          filename: /test_page_errors\.html/,
+          lineNumber: 1,
+          columnNumber: 23,
+          functionName: null,
+        },
+      ],
+      notes: null,
+      chromeContext: false,
+      isPromiseRejection: false,
+      isForwardedFromContentProcess: false,
+    },
+  ],
+  [
+    "([]).length = -1",
+    {
+      errorMessage: /array length/,
+      errorMessageName: "JSMSG_BAD_ARRAY_LENGTH",
+      sourceName: /test_page_errors/,
+      category: "content javascript",
+      timeStamp: FRACTIONAL_NUMBER_REGEX,
+      error: true,
+      warning: false,
+      info: false,
+      lineText: "",
+      lineNumber: NUMBER_REGEX,
+      columnNumber: NUMBER_REGEX,
+      exceptionDocURL: mdnUrl(
+        "docs/Web/JavaScript/Reference/Errors/Invalid_array_length"
+      ),
+      innerWindowID: NUMBER_REGEX,
+      private: false,
+      stacktrace: [
+        {
+          filename: /test_page_errors\.html/,
+          lineNumber: 1,
+          columnNumber: 2,
+          functionName: null,
+        },
+      ],
+      notes: null,
+      chromeContext: false,
+      isPromiseRejection: false,
+      isForwardedFromContentProcess: false,
+    },
+  ],
+  [
+    "'abc'.repeat(-1);",
+    {
+      errorMessage: /repeat count.*non-negative/,
+      errorMessageName: "JSMSG_NEGATIVE_REPETITION_COUNT",
+      sourceName: /test_page_errors/,
+      category: "content javascript",
+      timeStamp: FRACTIONAL_NUMBER_REGEX,
+      error: true,
+      warning: false,
+      info: false,
+      lineText: "",
+      lineNumber: NUMBER_REGEX,
+      columnNumber: NUMBER_REGEX,
+      exceptionDocURL: mdnUrl(
+        "docs/Web/JavaScript/Reference/Errors/Negative_repetition_count"
+      ),
+      innerWindowID: NUMBER_REGEX,
+      private: false,
+      stacktrace: [
+        {
+          filename: "self-hosted",
+          sourceId: null,
+          lineNumber: NUMBER_REGEX,
+          columnNumber: NUMBER_REGEX,
+          functionName: "repeat",
+        },
+        {
+          filename: /test_page_errors\.html/,
+          lineNumber: 1,
+          columnNumber: 7,
+          functionName: null,
+        },
+      ],
+      notes: null,
+      chromeContext: false,
+      isPromiseRejection: false,
+      isForwardedFromContentProcess: false,
+    },
+  ],
+  [
+    "'a'.repeat(2e28);",
+    {
+      errorMessage: /repeat count.*less than infinity/,
+      errorMessageName: "JSMSG_RESULTING_STRING_TOO_LARGE",
+      sourceName: /test_page_errors/,
+      category: "content javascript",
+      timeStamp: FRACTIONAL_NUMBER_REGEX,
+      error: true,
+      warning: false,
+      info: false,
+      lineText: "",
+      lineNumber: NUMBER_REGEX,
+      columnNumber: NUMBER_REGEX,
+      exceptionDocURL: mdnUrl(
+        "docs/Web/JavaScript/Reference/Errors/Resulting_string_too_large"
+      ),
+      innerWindowID: NUMBER_REGEX,
+      private: false,
+      stacktrace: [
+        {
+          filename: "self-hosted",
+          sourceId: null,
+          lineNumber: NUMBER_REGEX,
+          columnNumber: NUMBER_REGEX,
+          functionName: "repeat",
+        },
+        {
+          filename: /test_page_errors\.html/,
+          lineNumber: 1,
+          columnNumber: 5,
+          functionName: null,
+        },
+      ],
+      notes: null,
+      chromeContext: false,
+      isPromiseRejection: false,
+      isForwardedFromContentProcess: false,
+    },
+  ],
+  [
+    "77.1234.toExponential(-1);",
+    {
+      errorMessage: /out of range/,
+      errorMessageName: "JSMSG_PRECISION_RANGE",
+      sourceName: /test_page_errors/,
+      category: "content javascript",
+      timeStamp: FRACTIONAL_NUMBER_REGEX,
+      error: true,
+      warning: false,
+      info: false,
+      lineText: "",
+      lineNumber: NUMBER_REGEX,
+      columnNumber: NUMBER_REGEX,
+      exceptionDocURL: mdnUrl(
+        "docs/Web/JavaScript/Reference/Errors/Precision_range"
+      ),
+      innerWindowID: NUMBER_REGEX,
+      private: false,
+      stacktrace: [
+        {
+          filename: /test_page_errors\.html/,
+          lineNumber: 1,
+          columnNumber: 9,
+          functionName: null,
+        },
+      ],
+      notes: null,
+      chromeContext: false,
+      isPromiseRejection: false,
+      isForwardedFromContentProcess: false,
+    },
+  ],
+  [
+    "function a() { return; 1 + 1; }",
+    {
+      errorMessage: /unreachable code/,
+      errorMessageName: "JSMSG_STMT_AFTER_RETURN",
+      sourceName: /test_page_errors/,
+      category: "content javascript",
+      timeStamp: FRACTIONAL_NUMBER_REGEX,
+      error: false,
+      warning: true,
+      info: false,
+      sourceId: null,
+      lineText: "function a() { return; 1 + 1; }",
+      lineNumber: NUMBER_REGEX,
+      columnNumber: NUMBER_REGEX,
+      exceptionDocURL: mdnUrl(
+        "docs/Web/JavaScript/Reference/Errors/Stmt_after_return"
+      ),
+      innerWindowID: NUMBER_REGEX,
+      private: false,
+      stacktrace: null,
+      notes: null,
+      chromeContext: false,
+      isPromiseRejection: false,
+      isForwardedFromContentProcess: false,
+    },
+  ],
+  [
+    "{let a, a;}",
+    {
+      errorMessage: /redeclaration of/,
+      errorMessageName: "JSMSG_REDECLARED_VAR",
+      sourceName: /test_page_errors/,
+      category: "content javascript",
+      timeStamp: FRACTIONAL_NUMBER_REGEX,
+      error: true,
+      warning: false,
+      info: false,
+      sourceId: null,
+      lineText: "{let a, a;}",
+      lineNumber: NUMBER_REGEX,
+      columnNumber: NUMBER_REGEX,
+      exceptionDocURL: mdnUrl(
+        "docs/Web/JavaScript/Reference/Errors/Redeclared_parameter"
+      ),
+      innerWindowID: NUMBER_REGEX,
+      private: false,
+      stacktrace: [],
+      chromeContext: false,
+      isPromiseRejection: false,
+      isForwardedFromContentProcess: false,
+      notes: [
+        {
+          messageBody: /Previously declared at line/,
+          frame: {
+            source: /test_page_errors/,
+          },
+        },
+      ],
+    },
+  ],
+  [
+    `var error = new TypeError("abc");
+      error.name = "MyError";
+      error.message = "here";
+      throw error`,
+    {
+      errorMessage: /MyError: here/,
+      errorMessageName: "",
+      sourceName: /test_page_errors/,
+      category: "content javascript",
+      timeStamp: FRACTIONAL_NUMBER_REGEX,
+      error: true,
+      warning: false,
+      info: false,
+      lineText: "",
+      lineNumber: NUMBER_REGEX,
+      columnNumber: NUMBER_REGEX,
+      exceptionDocURL: undefined,
+      innerWindowID: NUMBER_REGEX,
+      private: false,
+      stacktrace: [
+        {
+          filename: /test_page_errors\.html/,
+          lineNumber: 1,
+          columnNumber: 13,
+          functionName: null,
+        },
+      ],
+      notes: null,
+      chromeContext: false,
+      isPromiseRejection: false,
+      isForwardedFromContentProcess: false,
+    },
+  ],
+  [
+    "DOMTokenList.prototype.contains.call([])",
+    {
+      errorMessage: /does not implement interface/,
+      errorMessageName: "MSG_METHOD_THIS_DOES_NOT_IMPLEMENT_INTERFACE",
+      sourceName: /test_page_errors/,
+      category: "content javascript",
+      timeStamp: FRACTIONAL_NUMBER_REGEX,
+      error: true,
+      warning: false,
+      info: false,
+      lineText: "",
+      lineNumber: NUMBER_REGEX,
+      columnNumber: NUMBER_REGEX,
+      exceptionDocURL: undefined,
+      innerWindowID: NUMBER_REGEX,
+      private: false,
+      stacktrace: [
+        {
+          filename: /test_page_errors\.html/,
+          lineNumber: 1,
+          columnNumber: 33,
+          functionName: null,
+        },
+      ],
+      notes: null,
+      chromeContext: false,
+      isPromiseRejection: false,
+      isForwardedFromContentProcess: false,
+    },
+  ],
+  [
+    `
+      function promiseThrow() {
+        var error2 = new TypeError("abc");
+        error2.name = "MyPromiseError";
+        error2.message = "here2";
+        return Promise.reject(error2);
+      }
+      promiseThrow()`,
+    {
+      errorMessage: /MyPromiseError: here2/,
+      errorMessageName: "",
+      sourceName: /test_page_errors/,
+      category: "content javascript",
+      timeStamp: FRACTIONAL_NUMBER_REGEX,
+      error: true,
+      warning: false,
+      info: false,
+      lineText: "",
+      lineNumber: NUMBER_REGEX,
+      columnNumber: NUMBER_REGEX,
+      exceptionDocURL: undefined,
+      innerWindowID: NUMBER_REGEX,
+      private: false,
+      stacktrace: [
+        {
+          filename: /test_page_errors\.html/,
+          sourceId: null,
+          lineNumber: 6,
+          columnNumber: 24,
+          functionName: "promiseThrow",
+        },
+        {
+          filename: /test_page_errors\.html/,
+          sourceId: null,
+          lineNumber: 8,
+          columnNumber: 7,
+          functionName: null,
+        },
+      ],
+      notes: null,
+      chromeContext: false,
+      isPromiseRejection: true,
+      isForwardedFromContentProcess: false,
+      [noUncaughtException]: true,
+    },
+  ],
+  [
+    // Error with a cause
+    `var originalError = new TypeError("abc");
+      var error = new Error("something went wrong", { cause: originalError })
+      throw error`,
+    {
+      errorMessage: /Error: something went wrong/,
+      errorMessageName: "",
+      sourceName: /test_page_errors/,
+      category: "content javascript",
+      timeStamp: FRACTIONAL_NUMBER_REGEX,
+      error: true,
+      warning: false,
+      info: false,
+      lineText: "",
+      lineNumber: NUMBER_REGEX,
+      columnNumber: NUMBER_REGEX,
+      innerWindowID: NUMBER_REGEX,
+      private: false,
+      stacktrace: [
+        {
+          filename: /test_page_errors\.html/,
+          lineNumber: 2,
+          columnNumber: 19,
+          functionName: null,
+        },
+      ],
+      exception: {
+        preview: {
+          cause: {
+            class: "TypeError",
+            preview: {
+              message: "abc",
+            },
+          },
+        },
+      },
+      notes: null,
+      chromeContext: false,
+      isPromiseRejection: false,
+      isForwardedFromContentProcess: false,
+    },
+  ],
+  [
+    // Error with a cause chain
+    `var a = new Error("err-a");
+     var b = new Error("err-b", { cause: a });
+     var c = new Error("err-c", { cause: b });
+     var d = new Error("err-d", { cause: c });
+      throw d`,
+    {
+      errorMessage: /Error: err-d/,
+      errorMessageName: "",
+      sourceName: /test_page_errors/,
+      category: "content javascript",
+      timeStamp: FRACTIONAL_NUMBER_REGEX,
+      error: true,
+      warning: false,
+      info: false,
+      lineText: "",
+      lineNumber: NUMBER_REGEX,
+      columnNumber: NUMBER_REGEX,
+      innerWindowID: NUMBER_REGEX,
+      private: false,
+      stacktrace: [
+        {
+          filename: /test_page_errors\.html/,
+          lineNumber: 4,
+          columnNumber: 14,
+          functionName: null,
+        },
+      ],
+      exception: {
+        preview: {
+          cause: {
+            class: "Error",
+            preview: {
+              message: "err-c",
+              cause: {
+                class: "Error",
+                preview: {
+                  message: "err-b",
+                  cause: {
+                    class: "Error",
+                    preview: {
+                      message: "err-a",
+                    },
+                  },
+                },
+              },
+            },
+          },
+        },
+      },
+      notes: null,
+      chromeContext: false,
+      isPromiseRejection: false,
+      isForwardedFromContentProcess: false,
+    },
+  ],
+  [
+    // Error with a null cause
+    `throw new Error("something went wrong", { cause: null })`,
+    {
+      errorMessage: /Error: something went wrong/,
+      errorMessageName: "",
+      sourceName: /test_page_errors/,
+      category: "content javascript",
+      timeStamp: FRACTIONAL_NUMBER_REGEX,
+      error: true,
+      warning: false,
+      info: false,
+      lineText: "",
+      lineNumber: NUMBER_REGEX,
+      columnNumber: NUMBER_REGEX,
+      innerWindowID: NUMBER_REGEX,
+      private: false,
+      stacktrace: [
+        {
+          filename: /test_page_errors\.html/,
+          lineNumber: 1,
+          columnNumber: 7,
+          functionName: null,
+        },
+      ],
+      exception: {
+        preview: {
+          cause: {
+            type: "null",
+          },
+        },
+      },
+      notes: null,
+      chromeContext: false,
+      isPromiseRejection: false,
+      isForwardedFromContentProcess: false,
+    },
+  ],
+  [
+    // Error with an undefined cause
+    `throw new Error("something went wrong", { cause: undefined })`,
+    {
+      errorMessage: /Error: something went wrong/,
+      errorMessageName: "",
+      sourceName: /test_page_errors/,
+      category: "content javascript",
+      timeStamp: FRACTIONAL_NUMBER_REGEX,
+      error: true,
+      warning: false,
+      info: false,
+      lineText: "",
+      lineNumber: NUMBER_REGEX,
+      columnNumber: NUMBER_REGEX,
+      innerWindowID: NUMBER_REGEX,
+      private: false,
+      stacktrace: [
+        {
+          filename: /test_page_errors\.html/,
+          lineNumber: 1,
+          columnNumber: 7,
+          functionName: null,
+        },
+      ],
+      exception: {
+        preview: {
+          cause: {
+            type: "undefined",
+          },
+        },
+      },
+      notes: null,
+      chromeContext: false,
+      isPromiseRejection: false,
+      isForwardedFromContentProcess: false,
+    },
+  ],
+  [
+    // Error with a number cause
+    `throw new Error("something went wrong", { cause: 0 })`,
+    {
+      errorMessage: /Error: something went wrong/,
+      errorMessageName: "",
+      sourceName: /test_page_errors/,
+      category: "content javascript",
+      timeStamp: FRACTIONAL_NUMBER_REGEX,
+      error: true,
+      warning: false,
+      info: false,
+      lineText: "",
+      lineNumber: NUMBER_REGEX,
+      columnNumber: NUMBER_REGEX,
+      innerWindowID: NUMBER_REGEX,
+      private: false,
+      stacktrace: [
+        {
+          filename: /test_page_errors\.html/,
+          lineNumber: 1,
+          columnNumber: 7,
+          functionName: null,
+        },
+      ],
+      exception: {
+        preview: {
+          cause: 0,
+        },
+      },
+      notes: null,
+      chromeContext: false,
+      isPromiseRejection: false,
+      isForwardedFromContentProcess: false,
+    },
+  ],
+  [
+    // Error with a string cause
+    `throw new Error("something went wrong", { cause: "ooops" })`,
+    {
+      errorMessage: /Error: something went wrong/,
+      errorMessageName: "",
+      sourceName: /test_page_errors/,
+      category: "content javascript",
+      timeStamp: FRACTIONAL_NUMBER_REGEX,
+      error: true,
+      warning: false,
+      info: false,
+      lineText: "",
+      lineNumber: NUMBER_REGEX,
+      columnNumber: NUMBER_REGEX,
+      innerWindowID: NUMBER_REGEX,
+      private: false,
+      stacktrace: [
+        {
+          filename: /test_page_errors\.html/,
+          lineNumber: 1,
+          columnNumber: 7,
+          functionName: null,
+        },
+      ],
+      exception: {
+        preview: {
+          cause: "ooops",
+        },
+      },
+      notes: null,
+      chromeContext: false,
+      isPromiseRejection: false,
+      isForwardedFromContentProcess: false,
+    },
+  ],
+]);
diff --git a/devtools/shared/commands/resource/tests/browser_resources_getAllResources.js b/devtools/shared/commands/resource/tests/browser_resources_getAllResources.js
new file mode 100644
index 0000000000..10bc8390d9
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/browser_resources_getAllResources.js
@@ -0,0 +1,128 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test getAllResources function of the ResourceCommand.
+
+const TEST_URI = "data:text/html;charset=utf-8,getAllResources test";
+
+add_task(async function () {
+  const tab = await addTab(TEST_URI);
+
+  const { client, resourceCommand, targetCommand } = await initResourceCommand(
+    tab
+  );
+
+  info("Check the resources gotten from getAllResources at initial");
+  is(
+    resourceCommand.getAllResources(resourceCommand.TYPES.CONSOLE_MESSAGE)
+      .length,
+    0,
+    "There is no resources at initial"
+  );
+
+  info(
+    "Start to watch the available resources in order to compare with resources gotten from getAllResources"
+  );
+  const availableResources = [];
+  const onAvailable = resources => availableResources.push(...resources);
+  await resourceCommand.watchResources(
+    [resourceCommand.TYPES.CONSOLE_MESSAGE],
+    { onAvailable }
+  );
+
+  info("Check the resources after some resources are available");
+  const messages = ["a", "b", "c"];
+  await logMessages(tab.linkedBrowser, messages);
+
+  try {
+    await waitFor(() => availableResources.length === messages.length);
+  } catch (e) {
+    ok(
+      false,
+      `Didn't receive the expected number of resources. Got ${
+        availableResources.length
+      }, expected ${messages.length} - ${availableResources
+        .map(r => r.message.arguments[0])
+        .join(" - ")}`
+    );
+  }
+
+  assertResources(
+    resourceCommand.getAllResources(resourceCommand.TYPES.CONSOLE_MESSAGE),
+    availableResources
+  );
+  assertResources(
+    resourceCommand.getAllResources(resourceCommand.TYPES.STYLESHEET),
+    []
+  );
+
+  info("Check the resources after reloading");
+  await BrowserTestUtils.reloadTab(tab);
+  assertResources(
+    resourceCommand.getAllResources(resourceCommand.TYPES.CONSOLE_MESSAGE),
+    []
+  );
+
+  info("Append some resources again to test unwatching");
+  const newMessages = ["d", "e", "f"];
+  await logMessages(tab.linkedBrowser, messages);
+  try {
+    await waitFor(
+      () =>
+        resourceCommand.getAllResources(resourceCommand.TYPES.CONSOLE_MESSAGE)
+          .length === newMessages.length
+    );
+  } catch (e) {
+    const resources = resourceCommand.getAllResources(
+      resourceCommand.TYPES.CONSOLE_MESSAGE
+    );
+    ok(
+      false,
+      `Didn't receive the expected number of resources. Got ${
+        resources.length
+      }, expected ${messages.length} - ${resources
+        .map(r => r.message.arguments.join(" | "))
+        .join(" - ")}`
+    );
+  }
+
+  info("Check the resources after unwatching");
+  resourceCommand.unwatchResources([resourceCommand.TYPES.CONSOLE_MESSAGE], {
+    onAvailable,
+  });
+  assertResources(
+    resourceCommand.getAllResources(resourceCommand.TYPES.CONSOLE_MESSAGE),
+    []
+  );
+
+  targetCommand.destroy();
+  await client.close();
+});
+
+function assertResources(resources, expectedResources) {
+  is(
+    resources.length,
+    expectedResources.length,
+    "Number of the resources is correct"
+  );
+
+  for (let i = 0; i < resources.length; i++) {
+    const resource = resources[i];
+    const expectedResource = expectedResources[i];
+    Assert.strictEqual(
+      resource,
+      expectedResource,
+      `The ${i}th resource is correct`
+    );
+  }
+}
+
+function logMessages(browser, messages) {
+  return SpecialPowers.spawn(browser, [messages], innerMessages => {
+    for (const message of innerMessages) {
+      content.console.log(message);
+    }
+  });
+}
diff --git a/devtools/shared/commands/resource/tests/browser_resources_invalid_api_usage.js b/devtools/shared/commands/resource/tests/browser_resources_invalid_api_usage.js
new file mode 100644
index 0000000000..8a1d809f04
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/browser_resources_invalid_api_usage.js
@@ -0,0 +1,76 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test watch/unwatchResources throw when provided with invalid types.
+
+const TEST_URI = "data:text/html;charset=utf-8,invalid api usage test";
+
+add_task(async function () {
+  const tab = await addTab(TEST_URI);
+
+  const { client, resourceCommand, targetCommand } = await initResourceCommand(
+    tab
+  );
+
+  const onAvailable = function () {};
+
+  await Assert.rejects(
+    resourceCommand.watchResources([null], { onAvailable }),
+    /ResourceCommand\.watchResources invoked with an unknown type/,
+    "watchResources should throw for null type"
+  );
+
+  await Assert.rejects(
+    resourceCommand.watchResources([undefined], { onAvailable }),
+    /ResourceCommand\.watchResources invoked with an unknown type/,
+    "watchResources should throw for undefined type"
+  );
+
+  await Assert.rejects(
+    resourceCommand.watchResources(["NOT_A_RESOURCE"], { onAvailable }),
+    /ResourceCommand\.watchResources invoked with an unknown type/,
+    "watchResources should throw for unknown type"
+  );
+
+  await Assert.rejects(
+    resourceCommand.watchResources(
+      [resourceCommand.TYPES.CONSOLE_MESSAGE, "NOT_A_RESOURCE"],
+      { onAvailable }
+    ),
+    /ResourceCommand\.watchResources invoked with an unknown type/,
+    "watchResources should throw for unknown type mixed with a correct type"
+  );
+
+  await Assert.throws(
+    () => resourceCommand.unwatchResources([null], { onAvailable }),
+    /ResourceCommand\.unwatchResources invoked with an unknown type/,
+    "unwatchResources should throw for null type"
+  );
+
+  await Assert.throws(
+    () => resourceCommand.unwatchResources([undefined], { onAvailable }),
+    /ResourceCommand\.unwatchResources invoked with an unknown type/,
+    "unwatchResources should throw for undefined type"
+  );
+
+  await Assert.throws(
+    () => resourceCommand.unwatchResources(["NOT_A_RESOURCE"], { onAvailable }),
+    /ResourceCommand\.unwatchResources invoked with an unknown type/,
+    "unwatchResources should throw for unknown type"
+  );
+
+  await Assert.throws(
+    () =>
+      resourceCommand.unwatchResources(
+        [resourceCommand.TYPES.CONSOLE_MESSAGE, "NOT_A_RESOURCE"],
+        { onAvailable }
+      ),
+    /ResourceCommand\.unwatchResources invoked with an unknown type/,
+    "unwatchResources should throw for unknown type mixed with a correct type"
+  );
+
+  targetCommand.destroy();
+  await client.close();
+});
diff --git a/devtools/shared/commands/resource/tests/browser_resources_last_private_context_exit.js b/devtools/shared/commands/resource/tests/browser_resources_last_private_context_exit.js
new file mode 100644
index 0000000000..1e2d894be3
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/browser_resources_last_private_context_exit.js
@@ -0,0 +1,94 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+// Verify that LAST_PRIVATE_CONTEXT_EXIT fires when closing the last opened private window
+
+"use strict";
+
+const NON_PRIVATE_TEST_URI =
+  "data:text/html;charset=utf8,<!DOCTYPE html>Not private";
+const PRIVATE_TEST_URI = `data:text/html;charset=utf8,<!DOCTYPE html>Test in private windows`;
+
+add_task(async function () {
+  await pushPref("devtools.browsertoolbox.scope", "everything");
+  const { commands } = await initMultiProcessResourceCommand();
+  const { resourceCommand } = commands;
+
+  const availableResources = [];
+  await resourceCommand.watchResources(
+    [resourceCommand.TYPES.LAST_PRIVATE_CONTEXT_EXIT],
+    {
+      onAvailable(resources) {
+        availableResources.push(resources);
+      },
+    }
+  );
+  is(
+    availableResources.length,
+    0,
+    "We do not get any LAST_PRIVATE_CONTEXT_EXIT after initialization"
+  );
+
+  await addTab(NON_PRIVATE_TEST_URI);
+
+  info("Open a new private window and select the new tab opened in it");
+  const privateWindow = await BrowserTestUtils.openNewBrowserWindow({
+    private: true,
+  });
+  ok(PrivateBrowsingUtils.isWindowPrivate(privateWindow), "window is private");
+  const privateBrowser = privateWindow.gBrowser;
+  privateBrowser.selectedTab = BrowserTestUtils.addTab(
+    privateBrowser,
+    PRIVATE_TEST_URI
+  );
+
+  info("private tab opened");
+  ok(
+    PrivateBrowsingUtils.isBrowserPrivate(privateBrowser.selectedBrowser),
+    "tab window is private"
+  );
+
+  info("Open a second tab in the private window");
+  await addTab(PRIVATE_TEST_URI, { window: privateWindow });
+
+  // Let a chance to an unexpected async event to be fired
+  await wait(1000);
+
+  is(
+    availableResources.length,
+    0,
+    "We do not get any LAST_PRIVATE_CONTEXT_EXIT when opening a private window"
+  );
+
+  info("Open a second private browsing window");
+  const secondPrivateWindow = await BrowserTestUtils.openNewBrowserWindow({
+    private: true,
+  });
+
+  info("Close the second private window");
+  secondPrivateWindow.BrowserTryToCloseWindow();
+
+  // Let a chance to an unexpected async event to be fired
+  await wait(1000);
+
+  is(
+    availableResources.length,
+    0,
+    "We do not get any LAST_PRIVATE_CONTEXT_EXIT when closing the second private window only"
+  );
+
+  info(
+    "close the private window and check if LAST_PRIVATE_CONTEXT_EXIT resource is sent"
+  );
+  privateWindow.BrowserTryToCloseWindow();
+
+  info("Wait for LAST_PRIVATE_CONTEXT_EXIT");
+  await waitFor(() => availableResources.length == 1);
+  is(
+    availableResources.length,
+    1,
+    "We get one LAST_PRIVATE_CONTEXT_EXIT when closing the last opened private window"
+  );
+
+  await commands.destroy();
+});
diff --git a/devtools/shared/commands/resource/tests/browser_resources_network_event_stacktraces.js b/devtools/shared/commands/resource/tests/browser_resources_network_event_stacktraces.js
new file mode 100644
index 0000000000..2200fcad9c
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/browser_resources_network_event_stacktraces.js
@@ -0,0 +1,100 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test the ResourceCommand API around NETWORK_EVENT_STACKTRACE
+
+const TEST_URI = `${URL_ROOT_SSL}network_document.html`;
+
+const REQUEST_STUB = {
+  code: `await fetch("/request_post_0.html", { method: "POST" });`,
+  expected: {
+    stacktraceAvailable: true,
+    lastFrame: {
+      filename:
+        "https://example.com/browser/devtools/shared/commands/resource/tests/network_document.html",
+      lineNumber: 1,
+      columnNumber: 40,
+      functionName: "triggerRequest",
+      asyncCause: null,
+    },
+  },
+};
+
+add_task(async function () {
+  info("Test network stacktraces events");
+  const tab = await addTab(TEST_URI);
+  const { client, resourceCommand, targetCommand } = await initResourceCommand(
+    tab
+  );
+
+  const networkEvents = new Map();
+  const stackTraces = new Map();
+
+  function onResourceAvailable(resources) {
+    for (const resource of resources) {
+      if (
+        resource.resourceType === resourceCommand.TYPES.NETWORK_EVENT_STACKTRACE
+      ) {
+        ok(
+          !networkEvents.has(resource.resourceId),
+          "The network event does not exist"
+        );
+
+        is(
+          resource.stacktraceAvailable,
+          REQUEST_STUB.expected.stacktraceAvailable,
+          "The stacktrace is available"
+        );
+        is(
+          JSON.stringify(resource.lastFrame),
+          JSON.stringify(REQUEST_STUB.expected.lastFrame),
+          "The last frame of the stacktrace is available"
+        );
+
+        stackTraces.set(resource.resourceId, true);
+        return;
+      }
+
+      if (resource.resourceType === resourceCommand.TYPES.NETWORK_EVENT) {
+        ok(
+          stackTraces.has(resource.stacktraceResourceId),
+          "The stack trace does exists"
+        );
+
+        networkEvents.set(resource.resourceId, true);
+      }
+    }
+  }
+
+  function onResourceUpdated() {}
+
+  await resourceCommand.watchResources(
+    [
+      resourceCommand.TYPES.NETWORK_EVENT_STACKTRACE,
+      resourceCommand.TYPES.NETWORK_EVENT,
+    ],
+    {
+      onAvailable: onResourceAvailable,
+      onUpdated: onResourceUpdated,
+    }
+  );
+
+  await triggerNetworkRequests(tab.linkedBrowser, [REQUEST_STUB.code]);
+
+  resourceCommand.unwatchResources(
+    [
+      resourceCommand.TYPES.NETWORK_EVENT_STACKTRACE,
+      resourceCommand.TYPES.NETWORK_EVENT,
+    ],
+    {
+      onAvailable: onResourceAvailable,
+      onUpdated: onResourceUpdated,
+    }
+  );
+
+  targetCommand.destroy();
+  await client.close();
+  BrowserTestUtils.removeTab(tab);
+});
diff --git a/devtools/shared/commands/resource/tests/browser_resources_network_events.js b/devtools/shared/commands/resource/tests/browser_resources_network_events.js
new file mode 100644
index 0000000000..da355fd023
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/browser_resources_network_events.js
@@ -0,0 +1,318 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test the ResourceCommand API around NETWORK_EVENT
+
+const ResourceCommand = require("resource://devtools/shared/commands/resource/resource-command.js");
+
+// We are borrowing tests from the netmonitor frontend
+const NETMONITOR_TEST_FOLDER =
+  "https://example.com/browser/devtools/client/netmonitor/test/";
+const CSP_URL = `${NETMONITOR_TEST_FOLDER}html_csp-test-page.html`;
+const JS_CSP_URL = `${NETMONITOR_TEST_FOLDER}js_websocket-worker-test.js`;
+const CSS_CSP_URL = `${NETMONITOR_TEST_FOLDER}internal-loaded.css`;
+
+const CSP_BLOCKED_REASON_CODE = 4000;
+
+add_task(async function testContentProcessRequests() {
+  info(`Tests for NETWORK_EVENT resources fired from the content process`);
+
+  const expectedAvailable = [
+    {
+      url: CSP_URL,
+      method: "GET",
+      isNavigationRequest: true,
+      chromeContext: false,
+    },
+    {
+      url: JS_CSP_URL,
+      method: "GET",
+      blockedReason: CSP_BLOCKED_REASON_CODE,
+      isNavigationRequest: false,
+      chromeContext: false,
+    },
+    {
+      url: CSS_CSP_URL,
+      method: "GET",
+      blockedReason: CSP_BLOCKED_REASON_CODE,
+      isNavigationRequest: false,
+      chromeContext: false,
+    },
+  ];
+  const expectedUpdated = [
+    {
+      url: CSP_URL,
+      method: "GET",
+      isNavigationRequest: true,
+      chromeContext: false,
+    },
+    {
+      url: JS_CSP_URL,
+      method: "GET",
+      blockedReason: CSP_BLOCKED_REASON_CODE,
+      isNavigationRequest: false,
+      chromeContext: false,
+    },
+    {
+      url: CSS_CSP_URL,
+      method: "GET",
+      blockedReason: CSP_BLOCKED_REASON_CODE,
+      isNavigationRequest: false,
+      chromeContext: false,
+    },
+  ];
+
+  await assertNetworkResourcesOnPage(
+    CSP_URL,
+    expectedAvailable,
+    expectedUpdated
+  );
+});
+
+add_task(async function testCanceledRequest() {
+  info(`Tests for NETWORK_EVENT resources with a canceled request`);
+
+  // Do a XHR request that we cancel against a slow loading page
+  const requestUrl =
+    "https://example.org/document-builder.sjs?delay=1000&html=foo";
+  const html =
+    "<!DOCTYPE html><script>(" +
+    function (xhrUrl) {
+      const xhr = new XMLHttpRequest();
+      xhr.open("GET", xhrUrl);
+      xhr.send(null);
+    } +
+    ")(" +
+    JSON.stringify(requestUrl) +
+    ")</script>";
+  const pageUrl =
+    "https://example.org/document-builder.sjs?html=" + encodeURIComponent(html);
+
+  const expectedAvailable = [
+    {
+      url: pageUrl,
+      method: "GET",
+      isNavigationRequest: true,
+      chromeContext: false,
+    },
+    {
+      url: requestUrl,
+      method: "GET",
+      isNavigationRequest: false,
+      blockedReason: "NS_BINDING_ABORTED",
+      chromeContext: false,
+    },
+  ];
+  const expectedUpdated = [
+    {
+      url: pageUrl,
+      method: "GET",
+      isNavigationRequest: true,
+      chromeContext: false,
+    },
+    {
+      url: requestUrl,
+      method: "GET",
+      isNavigationRequest: false,
+      blockedReason: "NS_BINDING_ABORTED",
+      chromeContext: false,
+    },
+  ];
+
+  // Register a one-off listener to cancel the XHR request
+  // Using XMLHttpRequest's abort() method from the content process
+  // isn't reliable and would introduce many race condition in the test.
+  // Canceling the request via nsIRequest.cancel privileged method,
+  // from the parent process is much more reliable.
+  const observer = {
+    QueryInterface: ChromeUtils.generateQI(["nsIObserver"]),
+    observe(subject, topic, data) {
+      subject = subject.QueryInterface(Ci.nsIHttpChannel);
+      if (subject.URI.spec == requestUrl) {
+        subject.cancel(Cr.NS_BINDING_ABORTED);
+        Services.obs.removeObserver(observer, "http-on-modify-request");
+      }
+    },
+  };
+  Services.obs.addObserver(observer, "http-on-modify-request");
+
+  await assertNetworkResourcesOnPage(
+    pageUrl,
+    expectedAvailable,
+    expectedUpdated
+  );
+});
+
+add_task(async function testIframeRequest() {
+  info(`Tests for NETWORK_EVENT resources with an iframe`);
+
+  // Do a XHR request that we cancel against a slow loading page
+  const iframeRequestUrl =
+    "https://example.org/document-builder.sjs?html=iframe-request";
+  const iframeHtml = `iframe<script>fetch("${iframeRequestUrl}")</script>`;
+  const iframeUrl =
+    "https://example.org/document-builder.sjs?html=" +
+    encodeURIComponent(iframeHtml);
+  const html = `top-document<iframe src="${iframeUrl}"></iframe>`;
+  const pageUrl =
+    "https://example.org/document-builder.sjs?html=" + encodeURIComponent(html);
+
+  const expectedAvailable = [
+    {
+      url: pageUrl,
+      method: "GET",
+      chromeContext: false,
+      isNavigationRequest: true,
+      // The top level navigation request relates to the previous top level target.
+      // Unfortunately, it is hard to test because it is racy.
+      // The target front might be destroyed and `targetFront.url` will be null.
+      // Or not just yet and be equal to "about:blank".
+    },
+    {
+      url: iframeUrl,
+      method: "GET",
+      isNavigationRequest: false,
+      targetFrontUrl: pageUrl,
+      chromeContext: false,
+    },
+    {
+      url: iframeRequestUrl,
+      method: "GET",
+      isNavigationRequest: false,
+      targetFrontUrl: iframeUrl,
+      chromeContext: false,
+    },
+  ];
+  const expectedUpdated = [
+    {
+      url: pageUrl,
+      method: "GET",
+      isNavigationRequest: true,
+      chromeContext: false,
+    },
+    {
+      url: iframeUrl,
+      method: "GET",
+      isNavigationRequest: false,
+      chromeContext: false,
+    },
+    {
+      url: iframeRequestUrl,
+      method: "GET",
+      isNavigationRequest: false,
+      chromeContext: false,
+    },
+  ];
+
+  await assertNetworkResourcesOnPage(
+    pageUrl,
+    expectedAvailable,
+    expectedUpdated
+  );
+});
+
+async function assertNetworkResourcesOnPage(
+  url,
+  expectedAvailable,
+  expectedUpdated
+) {
+  // First open a blank document to avoid spawning any request
+  const tab = await addTab("about:blank");
+
+  const commands = await CommandsFactory.forTab(tab);
+  await commands.targetCommand.startListening();
+  const { resourceCommand } = commands;
+
+  const onAvailable = resources => {
+    for (const resource of resources) {
+      // Immediately assert the resource, as the same resource object
+      // will be notified to onUpdated and so if we assert it later
+      // we will not highlight attributes that aren't set yet from onAvailable.
+      const idx = expectedAvailable.findIndex(e => e.url === resource.url);
+      Assert.notEqual(
+        idx,
+        -1,
+        "Found a matching available notification for: " + resource.url
+      );
+      // Remove the match from the list in case there is many requests with the same url
+      const [expected] = expectedAvailable.splice(idx, 1);
+
+      assertResources(resource, expected);
+    }
+  };
+  const onUpdated = updates => {
+    for (const { resource } of updates) {
+      const idx = expectedUpdated.findIndex(e => e.url === resource.url);
+      Assert.notEqual(
+        idx,
+        -1,
+        "Found a matching updated notification for: " + resource.url
+      );
+      // Remove the match from the list in case there is many requests with the same url
+      const [expected] = expectedUpdated.splice(idx, 1);
+
+      assertResources(resource, expected);
+    }
+  };
+
+  // Start observing for network events before loading the test page
+  await resourceCommand.watchResources([resourceCommand.TYPES.NETWORK_EVENT], {
+    onAvailable,
+    onUpdated,
+  });
+
+  // Load the test page that fires network requests
+  const onLoaded = BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser);
+  BrowserTestUtils.startLoadingURIString(gBrowser.selectedBrowser, url);
+  await onLoaded;
+
+  // Make sure we processed all the expected request updates
+  await waitFor(
+    () => !expectedAvailable.length,
+    "Wait for all expected available notifications"
+  );
+  await waitFor(
+    () => !expectedUpdated.length,
+    "Wait for all expected updated notifications"
+  );
+
+  resourceCommand.unwatchResources([resourceCommand.TYPES.NETWORK_EVENT], {
+    onAvailable,
+    onUpdated,
+  });
+
+  await commands.destroy();
+
+  BrowserTestUtils.removeTab(tab);
+}
+
+function assertResources(actual, expected) {
+  is(
+    actual.resourceType,
+    ResourceCommand.TYPES.NETWORK_EVENT,
+    "The resource type is correct"
+  );
+  is(
+    typeof actual.innerWindowId,
+    "number",
+    "All requests have an innerWindowId attribute"
+  );
+  ok(
+    actual.targetFront.isTargetFront,
+    "All requests have a targetFront attribute"
+  );
+
+  for (const name in expected) {
+    if (name == "targetFrontUrl") {
+      is(
+        actual.targetFront.url,
+        expected[name],
+        "The request matches the right target front"
+      );
+    } else {
+      is(actual[name], expected[name], `The '${name}' attribute is correct`);
+    }
+  }
+}
diff --git a/devtools/shared/commands/resource/tests/browser_resources_network_events_cache.js b/devtools/shared/commands/resource/tests/browser_resources_network_events_cache.js
new file mode 100644
index 0000000000..6708ef19e1
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/browser_resources_network_events_cache.js
@@ -0,0 +1,236 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test the ResourceCommand API internal cache / ignoreExistingResources around NETWORK_EVENT
+
+const ResourceCommand = require("resource://devtools/shared/commands/resource/resource-command.js");
+
+const EXAMPLE_DOMAIN = "https://example.com/";
+const TEST_URI = `${URL_ROOT_SSL}network_document.html`;
+
+add_task(async function () {
+  info("Test basic NETWORK_EVENT resources against ResourceCommand cache");
+  await testNetworkEventResourcesWithExistingResources();
+  await testNetworkEventResourcesWithoutExistingResources();
+});
+
+async function testNetworkEventResourcesWithExistingResources() {
+  info(`Tests for network event resources with the existing resources`);
+  await testNetworkEventResourcesWithCachedRequest({
+    ignoreExistingResources: false,
+    // 1 available event fired, for the existing resource in the cache.
+    // 1 available event fired, when live request is created.
+    expectedResourcesOnAvailable: {
+      [`${EXAMPLE_DOMAIN}cached_post.html`]: {
+        resourceType: ResourceCommand.TYPES.NETWORK_EVENT,
+        method: "POST",
+        isNavigationRequest: false,
+      },
+      [`${EXAMPLE_DOMAIN}live_get.html`]: {
+        resourceType: ResourceCommand.TYPES.NETWORK_EVENT,
+        method: "GET",
+        isNavigationRequest: false,
+      },
+    },
+    // 1 update events fired, when live request is updated.
+    expectedResourcesOnUpdated: {
+      [`${EXAMPLE_DOMAIN}live_get.html`]: {
+        resourceType: ResourceCommand.TYPES.NETWORK_EVENT,
+        method: "GET",
+      },
+    },
+  });
+}
+
+async function testNetworkEventResourcesWithoutExistingResources() {
+  info(`Tests for network event resources without the existing resources`);
+  await testNetworkEventResourcesWithCachedRequest({
+    ignoreExistingResources: true,
+    // 1 available event fired, when live request is created.
+    expectedResourcesOnAvailable: {
+      [`${EXAMPLE_DOMAIN}live_get.html`]: {
+        resourceType: ResourceCommand.TYPES.NETWORK_EVENT,
+        method: "GET",
+        isNavigationRequest: false,
+      },
+    },
+    // 1 update events fired, when live request is updated.
+    expectedResourcesOnUpdated: {
+      [`${EXAMPLE_DOMAIN}live_get.html`]: {
+        resourceType: ResourceCommand.TYPES.NETWORK_EVENT,
+        method: "GET",
+      },
+    },
+  });
+}
+
+/**
+ * This test helper is slightly complex as we workaround the fact
+ * that the server is not able to record network request done in the past.
+ * Because of that we have to start observer requests via ResourceCommand.watchResources
+ * before doing a request, and, before doing the actual call to watchResources
+ * we want to assert the behavior of.
+ */
+async function testNetworkEventResourcesWithCachedRequest(options) {
+  const tab = await addTab(TEST_URI);
+  const commands = await CommandsFactory.forTab(tab);
+  await commands.targetCommand.startListening();
+
+  const { resourceCommand } = commands;
+
+  info(
+    `Trigger some network requests *before* calling ResourceCommand.watchResources
+     in order to assert the behavior of already existing network events.`
+  );
+
+  // Register a first empty listener in order to ensure populating ResourceCommand
+  // internal cache of NETWORK_EVENT's. We can't retrieved past network requests
+  // when calling server's `watchResources`.
+  let resolveCachedRequestAvailable;
+  const onCachedRequestAvailable = new Promise(
+    r => (resolveCachedRequestAvailable = r)
+  );
+  const onAvailableToPopulateInternalCache = () => {};
+  const onUpdatedToPopulateInternalCache = resolveCachedRequestAvailable;
+  await resourceCommand.watchResources([resourceCommand.TYPES.NETWORK_EVENT], {
+    ignoreExistingResources: true,
+    onAvailable: onAvailableToPopulateInternalCache,
+    onUpdated: onUpdatedToPopulateInternalCache,
+  });
+
+  // We can only trigger the requests once `watchResources` settles,
+  // otherwise we might miss some events and they won't be present in the cache
+  const cachedRequest = `await fetch("/cached_post.html", { method: "POST" });`;
+  await triggerNetworkRequests(tab.linkedBrowser, [cachedRequest]);
+
+  // We have to ensure that ResourceCommand processed the Resource for this first
+  // cached request before calling watchResource a second time and report it.
+  // Wait for the updated notification to avoid receiving it during the next call
+  // to watchResources.
+  await onCachedRequestAvailable;
+
+  const actualResourcesOnAvailable = {};
+  const actualResourcesOnUpdated = {};
+
+  const {
+    expectedResourcesOnAvailable,
+    expectedResourcesOnUpdated,
+
+    ignoreExistingResources,
+  } = options;
+
+  const onAvailable = resources => {
+    for (const resource of resources) {
+      is(
+        resource.resourceType,
+        resourceCommand.TYPES.NETWORK_EVENT,
+        "Received a network event resource"
+      );
+      actualResourcesOnAvailable[resource.url] = resource;
+    }
+  };
+
+  const onUpdated = updates => {
+    for (const { resource } of updates) {
+      is(
+        resource.resourceType,
+        resourceCommand.TYPES.NETWORK_EVENT,
+        "Received a network update event resource"
+      );
+      actualResourcesOnUpdated[resource.url] = resource;
+    }
+  };
+
+  await resourceCommand.watchResources([resourceCommand.TYPES.NETWORK_EVENT], {
+    onAvailable,
+    onUpdated,
+    ignoreExistingResources,
+  });
+
+  info(
+    `Trigger the rest of the requests *after* calling ResourceCommand.watchResources
+     in order to assert the behavior of live network events.`
+  );
+  const liveRequest = `await fetch("/live_get.html", { method: "GET" });`;
+  await triggerNetworkRequests(tab.linkedBrowser, [liveRequest]);
+
+  info("Check the resources on available");
+
+  await waitUntil(
+    () =>
+      Object.keys(actualResourcesOnAvailable).length ==
+      Object.keys(expectedResourcesOnAvailable).length
+  );
+
+  is(
+    Object.keys(actualResourcesOnAvailable).length,
+    Object.keys(expectedResourcesOnAvailable).length,
+    "Got the expected number of network events fired onAvailable"
+  );
+
+  // assert the resources emitted when the network event is created
+  for (const key in expectedResourcesOnAvailable) {
+    const expected = expectedResourcesOnAvailable[key];
+    const actual = actualResourcesOnAvailable[key];
+    assertResources(actual, expected);
+  }
+
+  info("Check the resources on updated");
+
+  await waitUntil(
+    () =>
+      Object.keys(actualResourcesOnUpdated).length ==
+      Object.keys(expectedResourcesOnUpdated).length
+  );
+
+  is(
+    Object.keys(actualResourcesOnUpdated).length,
+    Object.keys(expectedResourcesOnUpdated).length,
+    "Got the expected number of network events fired onUpdated"
+  );
+
+  // assert the resources emitted when the network event is updated
+  for (const key in expectedResourcesOnUpdated) {
+    const expected = expectedResourcesOnUpdated[key];
+    const actual = actualResourcesOnUpdated[key];
+    assertResources(actual, expected);
+    // assert that the resourceId for the the available and updated events match
+    is(
+      actual.resourceId,
+      actualResourcesOnAvailable[key].resourceId,
+      `Available and update resource ids for ${key} are the same`
+    );
+  }
+
+  resourceCommand.unwatchResources([resourceCommand.TYPES.NETWORK_EVENT], {
+    onAvailable,
+    onUpdated,
+    ignoreExistingResources,
+  });
+
+  resourceCommand.unwatchResources([resourceCommand.TYPES.NETWORK_EVENT], {
+    onAvailable: onAvailableToPopulateInternalCache,
+  });
+
+  await commands.destroy();
+
+  BrowserTestUtils.removeTab(tab);
+}
+
+function assertResources(actual, expected) {
+  is(
+    actual.resourceType,
+    expected.resourceType,
+    "The resource type is correct"
+  );
+  is(actual.method, expected.method, "The method is correct");
+  if ("isNavigationRequest" in expected) {
+    is(
+      actual.isNavigationRequest,
+      expected.isNavigationRequest,
+      "The isNavigationRequest attribute is correct"
+    );
+  }
+}
diff --git a/devtools/shared/commands/resource/tests/browser_resources_network_events_navigation.js b/devtools/shared/commands/resource/tests/browser_resources_network_events_navigation.js
new file mode 100644
index 0000000000..44028318a2
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/browser_resources_network_events_navigation.js
@@ -0,0 +1,137 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test the ResourceCommand API around NETWORK_EVENT when navigating
+
+const TEST_URI = `${URL_ROOT_SSL}network_document_navigation.html`;
+const JS_URI = TEST_URI.replace(
+  "network_document_navigation.html",
+  "network_navigation.js"
+);
+
+add_task(async () => {
+  const tab = await addTab(TEST_URI);
+  const commands = await CommandsFactory.forTab(tab);
+  await commands.targetCommand.startListening();
+  const { resourceCommand } = commands;
+
+  const receivedResources = [];
+  const onAvailable = resources => {
+    for (const resource of resources) {
+      is(
+        resource.resourceType,
+        resourceCommand.TYPES.NETWORK_EVENT,
+        "Received a network event resource"
+      );
+      receivedResources.push(resource);
+    }
+  };
+  const onUpdated = updates => {
+    for (const { resource } of updates) {
+      is(
+        resource.resourceType,
+        resourceCommand.TYPES.NETWORK_EVENT,
+        "Received a network update event resource"
+      );
+    }
+  };
+
+  await resourceCommand.watchResources([resourceCommand.TYPES.NETWORK_EVENT], {
+    ignoreExistingResources: true,
+    onAvailable,
+    onUpdated,
+  });
+
+  await reloadBrowser();
+
+  await waitFor(() => receivedResources.length == 2);
+
+  const navigationRequest = receivedResources[0];
+  is(
+    navigationRequest.url,
+    TEST_URI,
+    "The first resource is for the navigation request"
+  );
+
+  const jsRequest = receivedResources[1];
+  is(jsRequest.url, JS_URI, "The second resource is for the javascript file");
+
+  async function getResponseContent(networkEvent) {
+    const packet = {
+      to: networkEvent.actor,
+      type: "getResponseContent",
+    };
+    const response = await commands.client.request(packet);
+    return response.content.text;
+  }
+
+  const HTML_CONTENT = await (await fetch(TEST_URI)).text();
+  const JS_CONTENT = await (await fetch(JS_URI)).text();
+
+  const htmlContent = await getResponseContent(navigationRequest);
+  is(htmlContent, HTML_CONTENT);
+  const jsContent = await getResponseContent(jsRequest);
+  is(jsContent, JS_CONTENT);
+
+  await reloadBrowser();
+
+  await waitFor(() => receivedResources.length == 4);
+
+  try {
+    await getResponseContent(navigationRequest);
+    ok(false, "Shouldn't work");
+  } catch (e) {
+    is(
+      e.error,
+      "noSuchActor",
+      "Without persist, we can't fetch previous document network data"
+    );
+  }
+
+  try {
+    await getResponseContent(jsRequest);
+    ok(false, "Shouldn't work");
+  } catch (e) {
+    is(
+      e.error,
+      "noSuchActor",
+      "Without persist, we can't fetch previous document network data"
+    );
+  }
+
+  const navigationRequest2 = receivedResources[2];
+  const jsRequest2 = receivedResources[3];
+  info("But we can fetch data for the last/new document");
+  const htmlContent2 = await getResponseContent(navigationRequest2);
+  is(htmlContent2, HTML_CONTENT);
+  const jsContent2 = await getResponseContent(jsRequest2);
+  is(jsContent2, JS_CONTENT);
+
+  info("Enable persist");
+  const networkParentFront =
+    await commands.watcherFront.getNetworkParentActor();
+  await networkParentFront.setPersist(true);
+
+  await reloadBrowser();
+
+  await waitFor(() => receivedResources.length == 6);
+
+  info("With persist, we can fetch previous document network data");
+  const htmlContent3 = await getResponseContent(navigationRequest2);
+  is(htmlContent3, HTML_CONTENT);
+  const jsContent3 = await getResponseContent(jsRequest2);
+  is(jsContent3, JS_CONTENT);
+
+  await resourceCommand.unwatchResources(
+    [resourceCommand.TYPES.NETWORK_EVENT],
+    {
+      onAvailable,
+      onUpdated,
+    }
+  );
+
+  await commands.destroy();
+  BrowserTestUtils.removeTab(tab);
+});
diff --git a/devtools/shared/commands/resource/tests/browser_resources_network_events_parent_process.js b/devtools/shared/commands/resource/tests/browser_resources_network_events_parent_process.js
new file mode 100644
index 0000000000..c5b3e436db
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/browser_resources_network_events_parent_process.js
@@ -0,0 +1,249 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+/**
+ * !! AFTER MOVING OR RENAMING THIS METHOD, UPDATE `EXPECTED` CONSTANTS BELOW !!
+ */
+const createParentProcessRequests = async () => {
+  info("Do some requests from the parent process");
+  // The line:column for `fetch` should be EXPECTED_REQUEST_LINE_1/COL_1
+  await fetch(FETCH_URI);
+
+  const img = new Image();
+  const onLoad = new Promise(r => img.addEventListener("load", r));
+  // The line:column for `img` below should be EXPECTED_REQUEST_LINE_2/COL_2
+  img.src = IMAGE_URI;
+  await onLoad;
+};
+
+const EXPECTED_METHOD_NAME = "createParentProcessRequests";
+const EXPECTED_REQUEST_LINE_1 = 12;
+const EXPECTED_REQUEST_COL_1 = 9;
+const EXPECTED_REQUEST_LINE_2 = 17;
+const EXPECTED_REQUEST_COL_2 = 3;
+
+// Test the ResourceCommand API around NETWORK_EVENT for the parent process
+
+const FETCH_URI = "https://example.com/document-builder.sjs?html=foo";
+// The img.src request gets cached regardless of `devtools.cache.disabled`.
+// Add a random parameter to the request to bypass the cache.
+const uuid = `${Date.now()}-${Math.random()}`;
+const IMAGE_URI = URL_ROOT_SSL + "test_image.png?" + uuid;
+
+add_task(async function testParentProcessRequests() {
+  // The test expects the main process commands instance to receive resources
+  // for content process requests.
+  await pushPref("devtools.browsertoolbox.scope", "everything");
+
+  const commands = await CommandsFactory.forMainProcess();
+  await commands.targetCommand.startListening();
+  const { resourceCommand } = commands;
+
+  const receivedNetworkEvents = [];
+  const receivedStacktraces = [];
+  const onAvailable = resources => {
+    for (const resource of resources) {
+      if (resource.resourceType == resourceCommand.TYPES.NETWORK_EVENT) {
+        receivedNetworkEvents.push(resource);
+      } else if (
+        resource.resourceType == resourceCommand.TYPES.NETWORK_EVENT_STACKTRACE
+      ) {
+        receivedStacktraces.push(resource);
+      }
+    }
+  };
+  const onUpdated = updates => {
+    for (const { resource } of updates) {
+      is(
+        resource.resourceType,
+        resourceCommand.TYPES.NETWORK_EVENT,
+        "Received a network update event resource"
+      );
+    }
+  };
+
+  await resourceCommand.watchResources(
+    [
+      resourceCommand.TYPES.NETWORK_EVENT,
+      resourceCommand.TYPES.NETWORK_EVENT_STACKTRACE,
+    ],
+    {
+      ignoreExistingResources: true,
+      onAvailable,
+      onUpdated,
+    }
+  );
+
+  await createParentProcessRequests();
+
+  const img2 = new Image();
+  img2.src = IMAGE_URI;
+
+  info("Wait for the network events");
+  await waitFor(() => receivedNetworkEvents.length == 3);
+  info("Wait for the network events stack traces");
+  // Note that we aren't getting any stacktrace for the second cached request
+  await waitFor(() => receivedStacktraces.length == 2);
+
+  info("Assert the fetch request");
+  const fetchRequest = receivedNetworkEvents[0];
+  is(
+    fetchRequest.url,
+    FETCH_URI,
+    "The first resource is for the fetch request"
+  );
+  ok(fetchRequest.chromeContext, "The fetch request is privileged");
+
+  const fetchStacktrace = receivedStacktraces[0].lastFrame;
+  is(receivedStacktraces[0].resourceId, fetchRequest.stacktraceResourceId);
+  is(fetchStacktrace.filename, gTestPath);
+  is(fetchStacktrace.lineNumber, EXPECTED_REQUEST_LINE_1);
+  is(fetchStacktrace.columnNumber, EXPECTED_REQUEST_COL_1);
+  is(fetchStacktrace.functionName, EXPECTED_METHOD_NAME);
+  is(fetchStacktrace.asyncCause, null);
+
+  async function getResponseContent(networkEvent) {
+    const packet = {
+      to: networkEvent.actor,
+      type: "getResponseContent",
+    };
+    const response = await commands.client.request(packet);
+    return response.content.text;
+  }
+
+  const fetchContent = await getResponseContent(fetchRequest);
+  is(fetchContent, "foo");
+
+  info("Assert the first image request");
+  const firstImageRequest = receivedNetworkEvents[1];
+  is(
+    firstImageRequest.url,
+    IMAGE_URI,
+    "The second resource is for the first image request"
+  );
+  ok(!firstImageRequest.fromCache, "The first image request isn't cached");
+  ok(firstImageRequest.chromeContext, "The first image request is privileged");
+
+  const firstImageStacktrace = receivedStacktraces[1].lastFrame;
+  is(receivedStacktraces[1].resourceId, firstImageRequest.stacktraceResourceId);
+  is(firstImageStacktrace.filename, gTestPath);
+  is(firstImageStacktrace.lineNumber, EXPECTED_REQUEST_LINE_2);
+  is(firstImageStacktrace.columnNumber, EXPECTED_REQUEST_COL_2);
+  is(firstImageStacktrace.functionName, EXPECTED_METHOD_NAME);
+  is(firstImageStacktrace.asyncCause, null);
+
+  info("Assert the second image request");
+  const secondImageRequest = receivedNetworkEvents[2];
+  is(
+    secondImageRequest.url,
+    IMAGE_URI,
+    "The third resource is for the second image request"
+  );
+  ok(secondImageRequest.fromCache, "The second image request is cached");
+  ok(
+    secondImageRequest.chromeContext,
+    "The second image request is privileged"
+  );
+
+  info(
+    "Open a content page to ensure we also receive request from content processes"
+  );
+  const pageUrl = "https://example.org/document-builder.sjs?html=foo";
+  const requestUrl = "https://example.org/document-builder.sjs?html=bar";
+  const tab = await addTab(pageUrl);
+
+  await waitFor(() => receivedNetworkEvents.length == 4);
+  const tabRequest = receivedNetworkEvents[3];
+  is(tabRequest.url, pageUrl, "The 4th resource is for the tab request");
+  ok(!tabRequest.chromeContext, "The 4th request is content");
+
+  info(
+    "Also spawn a privileged request from the content process, not bound to any WindowGlobal"
+  );
+  await SpecialPowers.spawn(
+    tab.linkedBrowser,
+    [requestUrl],
+    async function (uri) {
+      const { NetUtil } = ChromeUtils.importESModule(
+        "resource://gre/modules/NetUtil.sys.mjs"
+      );
+      const channel = NetUtil.newChannel({
+        uri,
+        loadUsingSystemPrincipal: true,
+      });
+      channel.open();
+    }
+  );
+  await removeTab(tab);
+
+  await waitFor(() => receivedNetworkEvents.length == 5);
+  const privilegedContentRequest = receivedNetworkEvents[4];
+  is(
+    privilegedContentRequest.url,
+    requestUrl,
+    "The 5th resource is for the privileged content process request"
+  );
+  ok(privilegedContentRequest.chromeContext, "The 5th request is privileged");
+
+  info("Now focus only on parent process resources");
+  await pushPref("devtools.browsertoolbox.scope", "parent-process");
+
+  info(
+    "Retrigger the two last requests. The tab document request and a privileged request. Both happening in the tab's content process."
+  );
+  const secondTab = await addTab(pageUrl);
+  await SpecialPowers.spawn(
+    secondTab.linkedBrowser,
+    [requestUrl],
+    async function (uri) {
+      const { NetUtil } = ChromeUtils.importESModule(
+        "resource://gre/modules/NetUtil.sys.mjs"
+      );
+      const channel = NetUtil.newChannel({
+        uri,
+        loadUsingSystemPrincipal: true,
+      });
+      channel.open();
+    }
+  );
+
+  await waitFor(() => receivedNetworkEvents.length == 6);
+
+  // nsIHttpChannel doesn't expose any attribute allowing to identify
+  // privileged requests done in content processes.
+  // Thus, preventing us from filtering them out correctly.
+  // Ideally, we would need some new attribute to know from which (content) process
+  // any channel originates from.
+  info(
+    "For now, we are still notified about the privileged content process request"
+  );
+  const secondPrivilegedContentRequest = receivedNetworkEvents[5];
+  is(
+    secondPrivilegedContentRequest.url,
+    requestUrl,
+    "The 6th resource is for the second privileged content process request"
+  );
+  ok(privilegedContentRequest.chromeContext, "The 6th request is privileged");
+
+  // Let some time to receive the tab request if that's not correctly filtered out
+  await wait(1000);
+  is(
+    receivedNetworkEvents.length,
+    6,
+    "But we don't receive the request for the tab request"
+  );
+
+  await removeTab(secondTab);
+
+  await resourceCommand.unwatchResources(
+    [resourceCommand.TYPES.NETWORK_EVENT],
+    {
+      onAvailable,
+      onUpdated,
+    }
+  );
+
+  await commands.destroy();
+});
diff --git a/devtools/shared/commands/resource/tests/browser_resources_platform_messages.js b/devtools/shared/commands/resource/tests/browser_resources_platform_messages.js
new file mode 100644
index 0000000000..4e74a97e38
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/browser_resources_platform_messages.js
@@ -0,0 +1,158 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test the ResourceCommand API around PLATFORM_MESSAGE
+// Reproduces assertions from: devtools/shared/webconsole/test/chrome/test_nsiconsolemessage.html
+
+add_task(async function () {
+  // Disable the preloaded process as it creates processes intermittently
+  // which forces the emission of RDP requests we aren't correctly waiting for.
+  await pushPref("dom.ipc.processPrelaunch.enabled", false);
+
+  await testPlatformMessagesResources();
+  await testPlatformMessagesResourcesWithIgnoreExistingResources();
+});
+
+async function testPlatformMessagesResources() {
+  const { client, resourceCommand, targetCommand } =
+    await initMultiProcessResourceCommand();
+
+  const cachedMessages = [
+    "This is a cached message",
+    "This is another cached message",
+  ];
+  const liveMessages = [
+    "This is a live message",
+    "This is another live message",
+  ];
+  const expectedMessages = [...cachedMessages, ...liveMessages];
+  const receivedMessages = [];
+
+  info(
+    "Log some messages *before* calling ResourceCommand.watchResources in order to assert the behavior of already existing messages."
+  );
+  Services.console.logStringMessage(expectedMessages[0]);
+  Services.console.logStringMessage(expectedMessages[1]);
+
+  let done;
+  const onAllMessagesReceived = new Promise(resolve => (done = resolve));
+  const onAvailable = resources => {
+    for (const resource of resources) {
+      if (!expectedMessages.includes(resource.message)) {
+        continue;
+      }
+
+      is(
+        resource.targetFront,
+        targetCommand.targetFront,
+        "The targetFront property is the expected one"
+      );
+
+      receivedMessages.push(resource.message);
+      is(
+        resource.message,
+        expectedMessages[receivedMessages.length - 1],
+        `Received the expected «${resource.message}» message, in the expected order`
+      );
+
+      // timeStamp are the result of a number in microsecond divided by 1000.
+      // so we can't expect a precise number of decimals, or even if there would
+      // be decimals at all.
+      ok(
+        resource.timeStamp.toString().match(/^\d+(\.\d{1,3})?$/),
+        `The resource has a timeStamp property ${resource.timeStamp}`
+      );
+
+      const isCachedMessage = receivedMessages.length <= cachedMessages.length;
+      is(
+        resource.isAlreadyExistingResource,
+        isCachedMessage,
+        "isAlreadyExistingResource has the expected value"
+      );
+
+      if (receivedMessages.length == expectedMessages.length) {
+        done();
+      }
+    }
+  };
+
+  await resourceCommand.watchResources(
+    [resourceCommand.TYPES.PLATFORM_MESSAGE],
+    {
+      onAvailable,
+    }
+  );
+
+  info(
+    "Now log messages *after* the call to ResourceCommand.watchResources and after having received all existing messages"
+  );
+  Services.console.logStringMessage(expectedMessages[2]);
+  Services.console.logStringMessage(expectedMessages[3]);
+
+  info("Waiting for all expected messages to be received");
+  await onAllMessagesReceived;
+  ok(true, "All the expected messages were received");
+
+  Services.console.reset();
+  targetCommand.destroy();
+  await client.close();
+}
+
+async function testPlatformMessagesResourcesWithIgnoreExistingResources() {
+  const { client, resourceCommand, targetCommand } =
+    await initMultiProcessResourceCommand();
+
+  info(
+    "Check whether onAvailable will not be called with existing platform messages"
+  );
+  const expectedMessages = ["This is 1st message", "This is 2nd message"];
+  Services.console.logStringMessage(expectedMessages[0]);
+  Services.console.logStringMessage(expectedMessages[1]);
+
+  const availableResources = [];
+  await resourceCommand.watchResources(
+    [resourceCommand.TYPES.PLATFORM_MESSAGE],
+    {
+      onAvailable: resources => {
+        for (const resource of resources) {
+          if (!expectedMessages.includes(resource.message)) {
+            continue;
+          }
+
+          availableResources.push(resource);
+        }
+      },
+      ignoreExistingResources: true,
+    }
+  );
+  is(
+    availableResources.length,
+    0,
+    "onAvailable wasn't called for existing platform messages"
+  );
+
+  info(
+    "Check whether onAvailable will be called with the future platform messages"
+  );
+  Services.console.logStringMessage(expectedMessages[0]);
+  Services.console.logStringMessage(expectedMessages[1]);
+
+  await waitUntil(() => availableResources.length === expectedMessages.length);
+  for (let i = 0; i < expectedMessages.length; i++) {
+    const resource = availableResources[i];
+    const { message } = resource;
+    const expected = expectedMessages[i];
+    is(message, expected, `Message[${i}] is correct`);
+    is(
+      resource.isAlreadyExistingResource,
+      false,
+      "isAlreadyExistingResource is false since we ignore existing resources"
+    );
+  }
+
+  Services.console.reset();
+  targetCommand.destroy();
+  await client.close();
+}
diff --git a/devtools/shared/commands/resource/tests/browser_resources_reflows.js b/devtools/shared/commands/resource/tests/browser_resources_reflows.js
new file mode 100644
index 0000000000..70242c826a
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/browser_resources_reflows.js
@@ -0,0 +1,115 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test the ResourceCommand API for reflows
+
+const {
+  TYPES,
+} = require("resource://devtools/shared/commands/resource/resource-command.js");
+
+add_task(async function () {
+  const tab = await addTab(
+    "https://example.com/document-builder.sjs?html=<h1>Test reflow resources</h1>"
+  );
+
+  const { client, resourceCommand, targetCommand } = await initResourceCommand(
+    tab
+  );
+
+  const resources = [];
+  const onAvailable = _resources => {
+    resources.push(..._resources);
+  };
+  await resourceCommand.watchResources([TYPES.REFLOW], {
+    onAvailable,
+  });
+
+  is(resources.length, 0, "No reflow resource were sent initially");
+
+  await SpecialPowers.spawn(tab.linkedBrowser, [], () => {
+    const el = content.document.createElement("div");
+    el.textContent = "1";
+    content.document.body.appendChild(el);
+  });
+
+  await waitFor(() => resources.length === 1);
+  checkReflowResource(resources[0]);
+
+  await SpecialPowers.spawn(tab.linkedBrowser, [], () => {
+    const el = content.document.querySelector("div");
+    el.style.display = "inline-grid";
+  });
+
+  await waitFor(() => resources.length === 2);
+  ok(
+    true,
+    "A reflow resource is sent when the display property of an element is modified"
+  );
+  checkReflowResource(resources.at(-1));
+
+  info("Check that adding an iframe does emit a reflow");
+  const iframeBC = await SpecialPowers.spawn(
+    tab.linkedBrowser,
+    [],
+    async () => {
+      const el = content.document.createElement("iframe");
+      const onIframeLoaded = new Promise(resolve =>
+        el.addEventListener("load", resolve, { once: true })
+      );
+      content.document.body.appendChild(el);
+      el.src =
+        "https://example.org/document-builder.sjs?html=<h2>remote iframe</h2>";
+      await onIframeLoaded;
+      return el.browsingContext;
+    }
+  );
+
+  await waitFor(() => resources.length === 3);
+  ok(true, "A reflow resource was received when adding a remote iframe");
+  checkReflowResource(resources.at(-1));
+
+  info("Check that we receive reflow resources for the remote iframe");
+  await SpecialPowers.spawn(iframeBC, [], () => {
+    const el = content.document.createElement("section");
+    el.textContent = "remote org iframe";
+    el.style.display = "grid";
+    content.document.body.appendChild(el);
+  });
+
+  await waitFor(() => resources.length === 4);
+  if (isFissionEnabled()) {
+    ok(
+      resources.at(-1).targetFront.url.includes("example.org"),
+      "The reflow resource is linked to the remote target"
+    );
+  }
+  checkReflowResource(resources.at(-1));
+
+  targetCommand.destroy();
+  await client.close();
+});
+
+function checkReflowResource(resource) {
+  is(
+    resource.resourceType,
+    TYPES.REFLOW,
+    "The resource has the expected resourceType"
+  );
+
+  ok(Array.isArray(resource.reflows), "the `reflows` property is an array");
+  for (const reflow of resource.reflows) {
+    is(
+      Number.isFinite(reflow.start),
+      true,
+      "reflow start property is a number"
+    );
+    is(Number.isFinite(reflow.end), true, "reflow end property is a number");
+    Assert.greaterOrEqual(
+      reflow.end,
+      reflow.start,
+      "end is greater than start"
+    );
+  }
+}
diff --git a/devtools/shared/commands/resource/tests/browser_resources_root_node.js b/devtools/shared/commands/resource/tests/browser_resources_root_node.js
new file mode 100644
index 0000000000..67ef5efd90
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/browser_resources_root_node.js
@@ -0,0 +1,129 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test the ResourceCommand API around ROOT_NODE
+
+/**
+ * The original test still asserts some scenarios using several watchRootNode
+ * call sites, which is not something we intend to support at the moment in the
+ * resource command.
+ *
+ * Otherwise this test checks the basic behavior of the resource when reloading
+ * an empty page.
+ */
+add_task(async function () {
+  // Open a test tab
+  const tab = await addTab("data:text/html,Root Node tests");
+
+  const { client, resourceCommand, targetCommand } = await initResourceCommand(
+    tab
+  );
+
+  const browser = gBrowser.selectedBrowser;
+
+  info("Call watchResources([ROOT_NODE], ...)");
+  let onAvailableCounter = 0;
+  const onAvailable = resources => (onAvailableCounter += resources.length);
+  await resourceCommand.watchResources([resourceCommand.TYPES.ROOT_NODE], {
+    onAvailable,
+  });
+
+  info("Wait until onAvailable has been called");
+  await waitUntil(() => onAvailableCounter === 1);
+  is(onAvailableCounter, 1, "onAvailable has been called 1 time");
+
+  info("Reload the selected browser");
+  browser.reload();
+
+  info(
+    "Wait until the watchResources([ROOT_NODE], ...) callback has been called"
+  );
+  await waitUntil(() => onAvailableCounter === 2);
+
+  is(onAvailableCounter, 2, "onAvailable has been called 2 times");
+
+  info("Call unwatchResources([ROOT_NODE], ...) for the onAvailable callback");
+  resourceCommand.unwatchResources([resourceCommand.TYPES.ROOT_NODE], {
+    onAvailable,
+  });
+
+  info("Reload the selected browser");
+  const reloaded = BrowserTestUtils.browserLoaded(browser);
+  browser.reload();
+  await reloaded;
+
+  is(
+    onAvailableCounter,
+    2,
+    "onAvailable was not called after calling unwatchResources"
+  );
+
+  // Cleanup
+  targetCommand.destroy();
+  await client.close();
+});
+
+/**
+ * Test that the watchRootNode API provides the expected node fronts.
+ */
+add_task(async function testRootNodeFrontIsCorrect() {
+  const tab = await addTab("data:text/html,<div id=div1>");
+
+  const { client, resourceCommand, targetCommand } = await initResourceCommand(
+    tab
+  );
+  const browser = gBrowser.selectedBrowser;
+
+  info("Call watchResources([ROOT_NODE], ...)");
+
+  let rootNodeResolve;
+  let rootNodePromise = new Promise(r => (rootNodeResolve = r));
+  const onAvailable = ([rootNodeFront]) => rootNodeResolve(rootNodeFront);
+  await resourceCommand.watchResources([resourceCommand.TYPES.ROOT_NODE], {
+    onAvailable,
+  });
+
+  info("Wait until onAvailable has been called");
+  const root1 = await rootNodePromise;
+  ok(!!root1, "onAvailable has been called with a valid argument");
+  is(
+    root1.resourceType,
+    resourceCommand.TYPES.ROOT_NODE,
+    "The resource has the expected type"
+  );
+
+  info("Check we can query an expected node under the retrieved root");
+  const div1 = await root1.walkerFront.querySelector(root1, "div");
+  is(div1.getAttribute("id"), "div1", "Correct root node retrieved");
+
+  info("Reload the selected browser");
+  rootNodePromise = new Promise(r => (rootNodeResolve = r));
+  browser.reload();
+
+  const root2 = await rootNodePromise;
+  Assert.notStrictEqual(
+    root1,
+    root2,
+    "onAvailable has been called with a different node front after reload"
+  );
+
+  info("Navigate to another URL");
+  rootNodePromise = new Promise(r => (rootNodeResolve = r));
+  BrowserTestUtils.startLoadingURIString(
+    browser,
+    `data:text/html,<div id=div3>`
+  );
+  const root3 = await rootNodePromise;
+  info("Check we can query an expected node under the retrieved root");
+  const div3 = await root3.walkerFront.querySelector(root3, "div");
+  is(div3.getAttribute("id"), "div3", "Correct root node retrieved");
+
+  // Cleanup
+  resourceCommand.unwatchResources([resourceCommand.TYPES.ROOT_NODE], {
+    onAvailable,
+  });
+  targetCommand.destroy();
+  await client.close();
+});
diff --git a/devtools/shared/commands/resource/tests/browser_resources_scope_flag.js b/devtools/shared/commands/resource/tests/browser_resources_scope_flag.js
new file mode 100644
index 0000000000..8537daf161
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/browser_resources_scope_flag.js
@@ -0,0 +1,128 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test that the ResourceCommand clears its pending events for resources emitted from
+// target destroyed when devtools.browsertoolbox.scope is updated.
+
+const TEST_URL =
+  "data:text/html;charset=utf-8," + encodeURIComponent(`<div id="test"></div>`);
+
+add_task(async function () {
+  // Do not run this test when both fission and EFT is disabled as it changes
+  // the number of targets
+  if (!isFissionEnabled() && !isEveryFrameTargetEnabled()) {
+    ok(true, "Don't go further is both Fission and EFT are disabled");
+    return;
+  }
+
+  // Disable the preloaded process as it gets created lazily and may interfere
+  // with process count assertions
+  await pushPref("dom.ipc.processPrelaunch.enabled", false);
+  // This preference helps destroying the content process when we close the tab
+  await pushPref("dom.ipc.keepProcessesAlive.web", 1);
+
+  // Start with multiprocess debugging enabled
+  await pushPref("devtools.browsertoolbox.scope", "everything");
+
+  const commands = await CommandsFactory.forMainProcess();
+  const targetCommand = commands.targetCommand;
+  await targetCommand.startListening();
+  const { TYPES } = targetCommand;
+
+  const targets = new Set();
+  const onAvailable = async ({ targetFront }) => {
+    targets.add(targetFront);
+  };
+  const onDestroyed = () => {};
+  await targetCommand.watchTargets({
+    types: [TYPES.PROCESS, TYPES.FRAME],
+    onAvailable,
+    onDestroyed,
+  });
+
+  info("Open a tab in a new content process");
+  const firstTab = await BrowserTestUtils.openNewForegroundTab({
+    gBrowser,
+    url: TEST_URL,
+    forceNewProcess: true,
+  });
+
+  const newTabInnerWindowId =
+    firstTab.linkedBrowser.browsingContext.currentWindowGlobal.innerWindowId;
+
+  info("Wait for the tab window global target");
+  const windowGlobalTarget = await waitFor(() =>
+    [...targets].find(
+      target =>
+        target.targetType == TYPES.FRAME &&
+        target.innerWindowId == newTabInnerWindowId
+    )
+  );
+
+  let gotTabResource = false;
+  const onResourceAvailable = resources => {
+    for (const resource of resources) {
+      if (resource.targetFront == windowGlobalTarget) {
+        gotTabResource = true;
+
+        if (resource.targetFront.isDestroyed()) {
+          ok(
+            false,
+            "we shouldn't get resources for the target that was destroyed when switching mode"
+          );
+        }
+      }
+    }
+  };
+
+  info("Start listening for resources");
+  await commands.resourceCommand.watchResources(
+    [commands.resourceCommand.TYPES.CONSOLE_MESSAGE],
+    {
+      onAvailable: onResourceAvailable,
+      ignoreExistingResources: true,
+    }
+  );
+
+  // Emit logs every ms to fill up the resourceCommand resource queue (pendingEvents)
+  const intervalId = await SpecialPowers.spawn(
+    firstTab.linkedBrowser,
+    [],
+    () => {
+      let counter = 0;
+      return content.wrappedJSObject.setInterval(() => {
+        counter++;
+        content.wrappedJSObject.console.log("STREAM_" + counter);
+      }, 1);
+    }
+  );
+
+  info("Wait until we get the first resource");
+  await waitFor(() => gotTabResource);
+
+  info("Disable multiprocess debugging");
+  await pushPref("devtools.browsertoolbox.scope", "parent-process");
+
+  info("Wait for the tab target to be destroyed");
+  await waitFor(() => windowGlobalTarget.isDestroyed());
+
+  info("Wait for a bit so any throttled action would have the time to occur");
+  await wait(1000);
+
+  // Stop listening for resources
+  await commands.resourceCommand.unwatchResources(
+    [commands.resourceCommand.TYPES.CONSOLE_MESSAGE],
+    {
+      onAvailable: onResourceAvailable,
+    }
+  );
+  // And stop the interval
+  await SpecialPowers.spawn(firstTab.linkedBrowser, [intervalId], id => {
+    content.wrappedJSObject.clearInterval(id);
+  });
+
+  targetCommand.destroy();
+  await commands.destroy();
+});
diff --git a/devtools/shared/commands/resource/tests/browser_resources_server_sent_events.js b/devtools/shared/commands/resource/tests/browser_resources_server_sent_events.js
new file mode 100644
index 0000000000..dab6c8d8cc
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/browser_resources_server_sent_events.js
@@ -0,0 +1,107 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test the ResourceCommand API around SERVER SENT EVENTS.
+
+const ResourceCommand = require("resource://devtools/shared/commands/resource/resource-command.js");
+
+const targets = {
+  TOP_LEVEL_DOCUMENT: "top-level-document",
+  IN_PROCESS_IFRAME: "in-process-frame",
+  OUT_PROCESS_IFRAME: "out-process-frame",
+};
+
+add_task(async function () {
+  info("Testing the top-level document");
+  await testServerSentEventResources(targets.TOP_LEVEL_DOCUMENT);
+  info("Testing the in-process iframe");
+  await testServerSentEventResources(targets.IN_PROCESS_IFRAME);
+  info("Testing the out-of-process iframe");
+  await testServerSentEventResources(targets.OUT_PROCESS_IFRAME);
+});
+
+async function testServerSentEventResources(target) {
+  const tab = await addTab(URL_ROOT_SSL + "sse_frontend.html");
+
+  const { client, resourceCommand, targetCommand } = await initResourceCommand(
+    tab
+  );
+
+  const availableResources = [];
+
+  function onResourceAvailable(resources) {
+    availableResources.push(...resources);
+  }
+
+  await resourceCommand.watchResources(
+    [resourceCommand.TYPES.SERVER_SENT_EVENT],
+    { onAvailable: onResourceAvailable }
+  );
+
+  openConnectionInContext(tab, target);
+
+  info("Check available resources");
+  // We expect only 2 resources
+  await waitUntil(() => availableResources.length === 2);
+
+  info("Check resource details");
+  // To make sure the channel id are the same
+  const httpChannelId = availableResources[0].httpChannelId;
+
+  ok(httpChannelId, "The channel id is set");
+  is(typeof httpChannelId, "number", "The channel id is a number");
+
+  assertResource(availableResources[0], {
+    messageType: "eventReceived",
+    httpChannelId,
+    data: {
+      payload: "Why so serious?",
+      eventName: "message",
+      lastEventId: "",
+      retry: 5000,
+    },
+  });
+
+  assertResource(availableResources[1], {
+    messageType: "eventSourceConnectionClosed",
+    httpChannelId,
+  });
+
+  await resourceCommand.unwatchResources(
+    [resourceCommand.TYPES.SERVER_SENT_EVENT],
+    { onAvailable: onResourceAvailable }
+  );
+
+  await targetCommand.destroy();
+  await client.close();
+  BrowserTestUtils.removeTab(tab);
+}
+
+function assertResource(resource, expected) {
+  is(
+    resource.resourceType,
+    ResourceCommand.TYPES.SERVER_SENT_EVENT,
+    "Resource type is correct"
+  );
+
+  checkObject(resource, expected);
+}
+
+async function openConnectionInContext(tab, target) {
+  let browsingContext = tab.linkedBrowser.browsingContext;
+  if (target !== targets.TOP_LEVEL_DOCUMENT) {
+    browsingContext = await SpecialPowers.spawn(
+      tab.linkedBrowser,
+      [target],
+      async _target => {
+        const iframe = content.document.getElementById(_target);
+        return iframe.browsingContext;
+      }
+    );
+  }
+  await SpecialPowers.spawn(browsingContext, [], async () => {
+    await content.wrappedJSObject.openConnection();
+  });
+}
diff --git a/devtools/shared/commands/resource/tests/browser_resources_several_resources.js b/devtools/shared/commands/resource/tests/browser_resources_several_resources.js
new file mode 100644
index 0000000000..c1a151e562
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/browser_resources_several_resources.js
@@ -0,0 +1,111 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+/**
+ * Check that the resource command is still properly watching for new targets
+ * after unwatching one resource, if there is still another watched resource.
+ */
+add_task(async function () {
+  // We will create a main process target list here in order to monitor
+  // resources from new tabs as they get created.
+  await pushPref("devtools.browsertoolbox.scope", "everything");
+
+  // Open a test tab
+  const tab = await addTab("data:text/html,Root Node tests");
+
+  const { client, resourceCommand, targetCommand } =
+    await initMultiProcessResourceCommand();
+
+  const { CONSOLE_MESSAGE, ROOT_NODE } = resourceCommand.TYPES;
+
+  // We are only interested in console messages as a resource, the ROOT_NODE one
+  // is here to test the ResourceCommand::unwatchResources API with several resources.
+  const receivedMessages = [];
+  const onAvailable = resources => {
+    for (const resource of resources) {
+      if (resource.resourceType === CONSOLE_MESSAGE) {
+        receivedMessages.push(resource);
+      }
+    }
+  };
+
+  info("Call watchResources([CONSOLE_MESSAGE, ROOT_NODE], ...)");
+  await resourceCommand.watchResources([CONSOLE_MESSAGE, ROOT_NODE], {
+    onAvailable,
+  });
+
+  info("Use console.log in the content page");
+  logInTab(tab, "test from data-url");
+  info(
+    "Wait until onAvailable received the CONSOLE_MESSAGE resource emitted from the data-url tab"
+  );
+  await waitUntil(() =>
+    receivedMessages.find(
+      resource => resource.message.arguments[0] === "test from data-url"
+    )
+  );
+
+  // Check that the resource command captures resources from new targets.
+  info("Open a first tab on the example.com domain");
+  const comTab = await addTab(
+    "https://example.com/document-builder.sjs?html=com"
+  );
+  info("Use console.log in the example.com page");
+  logInTab(comTab, "test-from-example-com");
+  info(
+    "Wait until onAvailable received the CONSOLE_MESSAGE resource emitted from the example.com tab"
+  );
+  await waitUntil(() =>
+    receivedMessages.find(
+      resource => resource.message.arguments[0] === "test-from-example-com"
+    )
+  );
+
+  info("Stop watching ROOT_NODE resources");
+  await resourceCommand.unwatchResources([ROOT_NODE], { onAvailable });
+
+  // Check that messages from new targets are still captured after calling
+  // unwatch for another resource.
+  info("Open a second tab on the example.org domain");
+  const orgTab = await addTab(
+    "https://example.org/document-builder.sjs?html=org"
+  );
+  info("Use console.log in the example.org page");
+  logInTab(orgTab, "test-from-example-org");
+  info(
+    "Wait until onAvailable received the CONSOLE_MESSAGE resource emitted from the example.org tab"
+  );
+  await waitUntil(() =>
+    receivedMessages.find(
+      resource => resource.message.arguments[0] === "test-from-example-org"
+    )
+  );
+
+  info("Stop watching CONSOLE_MESSAGE resources");
+  await resourceCommand.unwatchResources([CONSOLE_MESSAGE], { onAvailable });
+  await logInTab(tab, "test-again");
+
+  // We don't have a specific event to wait for here, so allow some time for
+  // the message to be received.
+  await wait(1000);
+
+  is(
+    receivedMessages.find(
+      resource => resource.message.arguments[0] === "test-again"
+    ),
+    undefined,
+    "The resource command should not watch CONSOLE_MESSAGE anymore"
+  );
+
+  // Cleanup
+  targetCommand.destroy();
+  await client.close();
+});
+
+function logInTab(tab, message) {
+  return ContentTask.spawn(tab.linkedBrowser, message, function (_message) {
+    content.console.log(_message);
+  });
+}
diff --git a/devtools/shared/commands/resource/tests/browser_resources_sources.js b/devtools/shared/commands/resource/tests/browser_resources_sources.js
new file mode 100644
index 0000000000..767f45283a
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/browser_resources_sources.js
@@ -0,0 +1,456 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test the ResourceCommand API around SOURCE.
+//
+// We cover each Spidermonkey Debugger Source's `introductionType`:
+// https://searchfox.org/mozilla-central/rev/4c184ca81b28f1ccffbfd08f465709b95bcb4aa1/js/src/doc/Debugger/Debugger.Source.md#172-213
+//
+// And especially cover sources being GC-ed before DevTools are opened
+// which are later recreated by `ThreadActor.resurrectSource`.
+
+const ResourceCommand = require("resource://devtools/shared/commands/resource/resource-command.js");
+
+const TEST_URL = URL_ROOT_SSL + "sources.html";
+
+const TEST_JS_URL = URL_ROOT_SSL + "sources.js";
+const TEST_WORKER_URL = URL_ROOT_SSL + "worker-sources.js";
+const TEST_SW_URL = URL_ROOT_SSL + "service-worker-sources.js";
+
+async function getExpectedResources(ignoreUnresurrectedSources = false) {
+  const htmlRequest = await fetch(TEST_URL);
+  const htmlContent = await htmlRequest.text();
+
+  // First list sources that aren't GC-ed, or that the thread actor is able to resurrect
+  const expectedSources = [
+    {
+      description: "eval",
+      sourceForm: {
+        introductionType: "eval",
+        sourceMapBaseURL: TEST_URL,
+        url: null,
+        isBlackBoxed: false,
+        sourceMapURL: null,
+        extensionName: null,
+        isInlineSource: false,
+      },
+      sourceContent: {
+        contentType: "text/javascript",
+        source: "this.global = function evalFunction() {}",
+      },
+    },
+    {
+      description: "new Function()",
+      sourceForm: {
+        introductionType: "Function",
+        sourceMapBaseURL: TEST_URL,
+        url: null,
+        isBlackBoxed: false,
+        sourceMapURL: null,
+        extensionName: null,
+        isInlineSource: false,
+      },
+      sourceContent: {
+        contentType: "text/javascript",
+        source: "function anonymous(\n) {\nreturn 42;\n}",
+      },
+    },
+    {
+      description: "Event Handler",
+      sourceForm: {
+        introductionType: "eventHandler",
+        sourceMapBaseURL: TEST_URL,
+        url: null,
+        isBlackBoxed: false,
+        sourceMapURL: null,
+        extensionName: null,
+        isInlineSource: false,
+      },
+      sourceContent: {
+        contentType: "text/javascript",
+        source: "console.log('link')",
+      },
+    },
+    {
+      description: "inline JS inserted at runtime",
+      sourceForm: {
+        introductionType: "scriptElement", // This is an injectedScript at SpiderMonkey level, but is translated into scriptElement by SourceActor.form()
+        sourceMapBaseURL: TEST_URL,
+        url: null,
+        isBlackBoxed: false,
+        sourceMapURL: null,
+        extensionName: null,
+        isInlineSource: false,
+      },
+      sourceContent: {
+        contentType: "text/javascript",
+        source: "console.log('inline-script')",
+      },
+    },
+    {
+      description: "inline JS",
+      sourceForm: {
+        introductionType: "scriptElement", // This is an inlineScript at SpiderMonkey level, but is translated into scriptElement by SourceActor.form()
+        sourceMapBaseURL: TEST_URL,
+        url: TEST_URL,
+        isBlackBoxed: false,
+        sourceMapURL: null,
+        extensionName: null,
+        isInlineSource: true,
+      },
+      sourceContent: {
+        contentType: "text/html",
+        source: htmlContent,
+      },
+    },
+    {
+      description: "worker script",
+      sourceForm: {
+        introductionType: undefined,
+        sourceMapBaseURL: TEST_WORKER_URL,
+        url: TEST_WORKER_URL,
+        isBlackBoxed: false,
+        sourceMapURL: null,
+        extensionName: null,
+        isInlineSource: false,
+      },
+      sourceContent: {
+        contentType: "text/javascript",
+        source: "/* eslint-disable */\nfunction workerSource() {}\n",
+      },
+    },
+    {
+      description: "service worker script",
+      sourceForm: {
+        introductionType: undefined,
+        sourceMapBaseURL: TEST_SW_URL,
+        url: TEST_SW_URL,
+        isBlackBoxed: false,
+        sourceMapURL: null,
+        extensionName: null,
+        isInlineSource: false,
+      },
+      sourceContent: {
+        contentType: "text/javascript",
+        source: "/* eslint-disable */\nfunction serviceWorkerSource() {}\n",
+      },
+    },
+    {
+      description: "independent js file",
+      sourceForm: {
+        introductionType: "scriptElement", // This is an srcScript at SpiderMonkey level, but is translated into scriptElement by SourceActor.form()
+        sourceMapBaseURL: TEST_JS_URL,
+        url: TEST_JS_URL,
+        isBlackBoxed: false,
+        sourceMapURL: null,
+        extensionName: null,
+        isInlineSource: false,
+      },
+      sourceContent: {
+        contentType: "text/javascript",
+        source: "/* eslint-disable */\nfunction scriptSource() {}\n",
+      },
+    },
+  ];
+
+  // Now list the sources that could be GC-ed for which the thread actor isn't able to resurrect.
+  // This is the sources that we can't assert when we fetch sources after the page is already loaded.
+  const unresurrectedSources = [
+    {
+      description: "DOM Timer",
+      sourceForm: {
+        introductionType: "domTimer",
+        sourceMapBaseURL: TEST_URL,
+        url: null,
+        isBlackBoxed: false,
+        sourceMapURL: null,
+        extensionName: null,
+        isInlineSource: false,
+      },
+      sourceContent: {
+        contentType: "text/javascript",
+        /* the domTimer is prefixed by many empty lines in order to be positioned at the same line
+           as in the HTML file where setTimeout is called.
+           This is probably done by SourceActor.actualText().
+           So the array size here, should be updated to match the line number of setTimeout call */
+        source: new Array(39).join("\n") + `console.log("timeout")`,
+      },
+    },
+    {
+      description: "javascript URL",
+      sourceForm: {
+        introductionType: "javascriptURL",
+        sourceMapBaseURL: isEveryFrameTargetEnabled()
+          ? "about:blank"
+          : TEST_URL,
+        url: null,
+        isBlackBoxed: false,
+        sourceMapURL: null,
+        extensionName: null,
+        isInlineSource: false,
+      },
+      sourceContent: {
+        contentType: "text/javascript",
+        source: "666",
+      },
+    },
+    {
+      description: "srcdoc attribute on iframes #1",
+      sourceForm: {
+        introductionType: "scriptElement",
+        // We do not assert url/sourceMapBaseURL as it includes the Debugger.Source.id
+        // which is random
+        isBlackBoxed: false,
+        sourceMapURL: null,
+        extensionName: null,
+        isInlineSource: false,
+      },
+      sourceContent: {
+        contentType: "text/javascript",
+        source: "console.log('srcdoc')",
+      },
+    },
+    {
+      description: "srcdoc attribute on iframes #2",
+      sourceForm: {
+        introductionType: "scriptElement",
+        // We do not assert url/sourceMapBaseURL as it includes the Debugger.Source.id
+        // which is random
+        isBlackBoxed: false,
+        sourceMapURL: null,
+        extensionName: null,
+        isInlineSource: false,
+      },
+      sourceContent: {
+        contentType: "text/javascript",
+        source: "console.log('srcdoc 2')",
+      },
+    },
+  ];
+
+  if (ignoreUnresurrectedSources) {
+    return expectedSources;
+  }
+  return expectedSources.concat(unresurrectedSources);
+}
+
+add_task(async function testSourcesOnload() {
+  // Load an blank document first, in order to load the test page only once we already
+  // started watching for sources
+  const tab = await addTab("about:blank");
+
+  const commands = await CommandsFactory.forTab(tab);
+  const { targetCommand, resourceCommand } = commands;
+
+  // Force the target list to cover workers and debug all the targets
+  targetCommand.listenForWorkers = true;
+  targetCommand.listenForServiceWorkers = true;
+  await targetCommand.startListening();
+
+  info("Check already available resources");
+  const availableResources = [];
+  await resourceCommand.watchResources([resourceCommand.TYPES.SOURCE], {
+    onAvailable: resources => availableResources.push(...resources),
+  });
+
+  const promiseLoad = BrowserTestUtils.browserLoaded(
+    gBrowser.selectedBrowser,
+    false,
+    TEST_URL
+  );
+  BrowserTestUtils.startLoadingURIString(tab.linkedBrowser, TEST_URL);
+  await promiseLoad;
+
+  // Some sources may be created after the document is done loading (like eventHandler usecase)
+  // so we may be received *after* watchResource resolved
+  const expectedResources = await getExpectedResources();
+  await waitFor(
+    () => availableResources.length >= expectedResources.length,
+    "Got all the sources"
+  );
+
+  await assertResources(availableResources, expectedResources);
+
+  await commands.destroy();
+
+  await SpecialPowers.spawn(tab.linkedBrowser, [], async () => {
+    // registrationPromise is set by the test page.
+    const registration = await content.wrappedJSObject.registrationPromise;
+    registration.unregister();
+  });
+});
+
+add_task(async function testGarbagedCollectedSources() {
+  info(
+    "Assert SOURCES on an already loaded page with some sources that have been GC-ed"
+  );
+  const tab = await addTab(TEST_URL);
+
+  info("Force some GC to free some sources");
+  await SpecialPowers.spawn(tab.linkedBrowser, [], async () => {
+    Cu.forceGC();
+    Cu.forceCC();
+  });
+
+  const commands = await CommandsFactory.forTab(tab);
+  const { targetCommand, resourceCommand } = commands;
+
+  // Force the target list to cover workers and debug all the targets
+  targetCommand.listenForWorkers = true;
+  targetCommand.listenForServiceWorkers = true;
+  await targetCommand.startListening();
+
+  info("Check already available resources");
+  const availableResources = [];
+  await resourceCommand.watchResources([resourceCommand.TYPES.SOURCE], {
+    onAvailable: resources => availableResources.push(...resources),
+  });
+
+  // Some sources may be created after the document is done loading (like eventHandler usecase)
+  // so we may be received *after* watchResource resolved
+  const expectedResources = await getExpectedResources(true);
+  await waitFor(
+    () => availableResources.length >= expectedResources.length,
+    "Got all the sources"
+  );
+
+  await assertResources(availableResources, expectedResources);
+
+  await commands.destroy();
+
+  await SpecialPowers.spawn(tab.linkedBrowser, [], async () => {
+    // registrationPromise is set by the test page.
+    const registration = await content.wrappedJSObject.registrationPromise;
+    registration.unregister();
+  });
+});
+
+/**
+ * Assert that evaluating sources for a new global, in the parent process
+ * using the shared system principal will spawn SOURCE resources.
+ *
+ * For this we use a special `commands` which replicate what browser console
+ * and toolbox use.
+ */
+add_task(async function testParentProcessPrivilegedSources() {
+  // Use a custom loader + server + client in order to spawn the server
+  // in a distinct system compartment, so that it can see the system compartment
+  // sandbox we are about to create in this test
+  const client = await CommandsFactory.spawnClientToDebugSystemPrincipal();
+
+  const commands = await CommandsFactory.forMainProcess({ client });
+  await commands.targetCommand.startListening();
+  const { resourceCommand } = commands;
+
+  info("Check already available resources");
+  const availableResources = [];
+  await resourceCommand.watchResources([resourceCommand.TYPES.SOURCE], {
+    onAvailable: resources => availableResources.push(...resources),
+  });
+  ok(
+    !!availableResources.length,
+    "We get many sources reported from a multiprocess command"
+  );
+
+  // Clear the list of sources
+  availableResources.length = 0;
+
+  // Force the creation of a new privileged source
+  const systemPrincipal = Cc["@mozilla.org/systemprincipal;1"].createInstance(
+    Ci.nsIPrincipal
+  );
+  const sandbox = Cu.Sandbox(systemPrincipal);
+  Cu.evalInSandbox("function foo() {}", sandbox, null, "http://foo.com");
+
+  info("Wait for the sandbox source");
+  await waitFor(() => {
+    return availableResources.some(
+      resource => resource.url == "http://foo.com/"
+    );
+  });
+
+  const expectedResources = [
+    {
+      description: "privileged sandbox script",
+      sourceForm: {
+        introductionType: undefined,
+        sourceMapBaseURL: "http://foo.com/",
+        url: "http://foo.com/",
+        isBlackBoxed: false,
+        sourceMapURL: null,
+        extensionName: null,
+        isInlineSource: false,
+      },
+      sourceContent: {
+        contentType: "text/javascript",
+        source: "function foo() {}",
+      },
+    },
+  ];
+  const matchingResource = availableResources.filter(resource =>
+    resource.url.includes("http://foo.com")
+  );
+  await assertResources(matchingResource, expectedResources);
+
+  await commands.destroy();
+});
+
+async function assertResources(resources, expected) {
+  is(
+    resources.length,
+    expected.length,
+    "Length of existing resources is correct at initial"
+  );
+  for (let i = 0; i < resources.length; i++) {
+    await assertResource(resources[i], expected);
+  }
+}
+
+async function assertResource(source, expected) {
+  is(
+    source.resourceType,
+    ResourceCommand.TYPES.SOURCE,
+    "Resource type is correct"
+  );
+
+  const threadFront = await source.targetFront.getFront("thread");
+  // `source` is SourceActor's form()
+  // so try to instantiate the related SourceFront:
+  const sourceFront = threadFront.source(source);
+  // then fetch source content
+  const sourceContent = await sourceFront.source();
+
+  // Order of sources is random, so we have to find the best expected resource.
+  // The only unique attribute is the JS Source text content.
+  const matchingExpected = expected.find(res => {
+    return res.sourceContent.source == sourceContent.source;
+  });
+  ok(
+    matchingExpected,
+    `This source was expected with source content being "${sourceContent.source}"`
+  );
+  info(`Found "#${matchingExpected.description}"`);
+  assertObject(
+    sourceContent,
+    matchingExpected.sourceContent,
+    matchingExpected.description
+  );
+
+  assertObject(
+    source,
+    matchingExpected.sourceForm,
+    matchingExpected.description
+  );
+}
+
+function assertObject(object, expected, description) {
+  for (const field in expected) {
+    is(
+      object[field],
+      expected[field],
+      `The value of ${field} is correct for "#${description}"`
+    );
+  }
+}
diff --git a/devtools/shared/commands/resource/tests/browser_resources_stylesheets.js b/devtools/shared/commands/resource/tests/browser_resources_stylesheets.js
new file mode 100644
index 0000000000..ec81e8118d
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/browser_resources_stylesheets.js
@@ -0,0 +1,713 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test the ResourceCommand API around STYLESHEET.
+
+const ResourceCommand = require("resource://devtools/shared/commands/resource/resource-command.js");
+
+const STYLE_TEST_URL = URL_ROOT_SSL + "style_document.html";
+
+const EXISTING_RESOURCES = [
+  {
+    styleText: "body { color: lime; }",
+    href: null,
+    nodeHref:
+      "https://example.com/browser/devtools/shared/commands/resource/tests/style_document.html",
+    isNew: false,
+    disabled: false,
+    constructed: false,
+    ruleCount: 1,
+    atRules: [],
+  },
+  {
+    styleText: "body { margin: 1px; }",
+    href: "https://example.com/browser/devtools/shared/commands/resource/tests/style_document.css",
+    nodeHref:
+      "https://example.com/browser/devtools/shared/commands/resource/tests/style_document.html",
+    isNew: false,
+    disabled: false,
+    constructed: false,
+    ruleCount: 1,
+    atRules: [],
+  },
+  {
+    styleText: "",
+    href: null,
+    nodeHref: null,
+    isNew: false,
+    disabled: false,
+    constructed: true,
+    ruleCount: 1,
+    atRules: [],
+  },
+  {
+    styleText: "body { background-color: pink; }",
+    href: null,
+    nodeHref:
+      "https://example.org/browser/devtools/shared/commands/resource/tests/style_iframe.html",
+    isNew: false,
+    disabled: false,
+    constructed: false,
+    ruleCount: 1,
+    atRules: [],
+  },
+  {
+    styleText: "body { padding: 1px; }",
+    href: "https://example.org/browser/devtools/shared/commands/resource/tests/style_iframe.css",
+    nodeHref:
+      "https://example.org/browser/devtools/shared/commands/resource/tests/style_iframe.html",
+    isNew: false,
+    disabled: false,
+    constructed: false,
+    ruleCount: 1,
+    atRules: [],
+  },
+];
+
+const ADDITIONAL_INLINE_RESOURCE = {
+  styleText:
+    "@media all { body { color: red; } } @media print { body { color: cyan; } } body { font-size: 10px; }",
+  href: null,
+  nodeHref:
+    "https://example.com/browser/devtools/shared/commands/resource/tests/style_document.html",
+  isNew: false,
+  disabled: false,
+  constructed: false,
+  ruleCount: 5,
+  atRules: [
+    {
+      type: "media",
+      conditionText: "all",
+      matches: true,
+      line: 1,
+      column: 1,
+    },
+    {
+      type: "media",
+      conditionText: "print",
+      matches: false,
+      line: 1,
+      column: 37,
+    },
+  ],
+};
+
+const ADDITIONAL_CONSTRUCTED_RESOURCE = {
+  styleText: "",
+  href: null,
+  nodeHref: null,
+  isNew: false,
+  disabled: false,
+  constructed: true,
+  ruleCount: 2,
+  atRules: [],
+};
+
+const ADDITIONAL_FROM_ACTOR_RESOURCE = {
+  styleText: "body { font-size: 10px; }",
+  href: null,
+  nodeHref:
+    "https://example.com/browser/devtools/shared/commands/resource/tests/style_document.html",
+  isNew: true,
+  disabled: false,
+  constructed: false,
+  ruleCount: 1,
+  atRules: [],
+};
+
+add_task(async function () {
+  await testResourceAvailableDestroyedFeature();
+  await testResourceUpdateFeature();
+  await testNestedResourceUpdateFeature();
+});
+
+function pushAvailableResource(availableResources) {
+  // TODO(bug 1826538): Find a better way of dealing with these.
+  return function (resources) {
+    for (const resource of resources) {
+      if (resource.href?.startsWith("resource://")) {
+        continue;
+      }
+      availableResources.push(resource);
+    }
+  };
+}
+
+async function testResourceAvailableDestroyedFeature() {
+  info("Check resource available feature of the ResourceCommand");
+
+  const tab = await addTab(STYLE_TEST_URL);
+  let resourceTimingEntryCounts = await getResourceTimingCount(tab);
+  is(
+    resourceTimingEntryCounts,
+    2,
+    "Should have two entires for resource timing"
+  );
+
+  const { client, resourceCommand, targetCommand } = await initResourceCommand(
+    tab
+  );
+
+  info("Check whether ResourceCommand gets existing stylesheet");
+  const availableResources = [];
+  const destroyedResources = [];
+  await resourceCommand.watchResources([resourceCommand.TYPES.STYLESHEET], {
+    onAvailable: pushAvailableResource(availableResources),
+    onDestroyed: resources => destroyedResources.push(...resources),
+  });
+
+  is(
+    availableResources.length,
+    EXISTING_RESOURCES.length,
+    "Length of existing resources is correct"
+  );
+  for (let i = 0; i < availableResources.length; i++) {
+    const availableResource = availableResources[i];
+    // We can not expect the resources to always be forwarded in the same order.
+    // See intermittent Bug 1655016.
+    const expectedResource = findMatchingExpectedResource(availableResource);
+    ok(expectedResource, "Found a matching expected resource for the resource");
+    await assertResource(availableResource, expectedResource);
+  }
+
+  resourceTimingEntryCounts = await getResourceTimingCount(tab);
+  is(
+    resourceTimingEntryCounts,
+    2,
+    "Should still have two entires for resource timing after devtools APIs have been triggered"
+  );
+
+  info("Check whether ResourceCommand gets additonal stylesheet");
+  await ContentTask.spawn(
+    tab.linkedBrowser,
+    ADDITIONAL_INLINE_RESOURCE.styleText,
+    text => {
+      const document = content.document;
+      const stylesheet = document.createElement("style");
+      stylesheet.id = "inline-from-test";
+      stylesheet.textContent = text;
+      document.body.appendChild(stylesheet);
+    }
+  );
+  await waitUntil(
+    () => availableResources.length === EXISTING_RESOURCES.length + 1
+  );
+  await assertResource(
+    availableResources[availableResources.length - 1],
+    ADDITIONAL_INLINE_RESOURCE
+  );
+
+  info("Check whether ResourceCommand gets additonal constructed stylesheet");
+  await ContentTask.spawn(tab.linkedBrowser, null, () => {
+    const document = content.document;
+    const s = new content.CSSStyleSheet();
+    // We use the different number of rules to meaningfully differentiate
+    // between constructed stylesheets.
+    s.replaceSync("foo { color: red } bar { color: blue }");
+    // TODO(bug 1751346): wrappedJSObject should be unnecessary.
+    document.wrappedJSObject.adoptedStyleSheets.push(s);
+  });
+  await waitUntil(
+    () => availableResources.length === EXISTING_RESOURCES.length + 2
+  );
+  await assertResource(
+    availableResources[availableResources.length - 1],
+    ADDITIONAL_CONSTRUCTED_RESOURCE
+  );
+
+  info(
+    "Check whether ResourceCommand gets additonal stylesheet which is added by DevTools"
+  );
+  const styleSheetsFront = await targetCommand.targetFront.getFront(
+    "stylesheets"
+  );
+  await styleSheetsFront.addStyleSheet(
+    ADDITIONAL_FROM_ACTOR_RESOURCE.styleText
+  );
+  await waitUntil(
+    () => availableResources.length === EXISTING_RESOURCES.length + 3
+  );
+  await assertResource(
+    availableResources[availableResources.length - 1],
+    ADDITIONAL_FROM_ACTOR_RESOURCE
+  );
+
+  info("Check resource destroyed feature of the ResourceCommand");
+  is(destroyedResources.length, 0, "There was no removed stylesheets yet");
+
+  info("Remove inline stylesheet added in the test");
+  await ContentTask.spawn(tab.linkedBrowser, null, () => {
+    content.document.querySelector("#inline-from-test").remove();
+  });
+  await waitUntil(() => destroyedResources.length === 1);
+  assertDestroyed(destroyedResources[0], {
+    resourceId: availableResources.at(-3).resourceId,
+  });
+
+  info("Remove existing top-level inline stylesheet");
+  await ContentTask.spawn(tab.linkedBrowser, null, () => {
+    content.document.querySelector("style").remove();
+  });
+  await waitUntil(() => destroyedResources.length === 2);
+  assertDestroyed(destroyedResources[1], {
+    resourceId: availableResources.find(
+      resource =>
+        findMatchingExpectedResource(resource) === EXISTING_RESOURCES[0]
+    ).resourceId,
+  });
+
+  info("Remove existing top-level <link> stylesheet");
+  await ContentTask.spawn(tab.linkedBrowser, null, () => {
+    content.document.querySelector("link").remove();
+  });
+  await waitUntil(() => destroyedResources.length === 3);
+  assertDestroyed(destroyedResources[2], {
+    resourceId: availableResources.find(
+      resource =>
+        findMatchingExpectedResource(resource) === EXISTING_RESOURCES[1]
+    ).resourceId,
+  });
+
+  info("Remove existing iframe inline stylesheet");
+  const iframeBrowsingContext = await SpecialPowers.spawn(
+    tab.linkedBrowser,
+    [],
+    () => content.document.querySelector("iframe").browsingContext
+  );
+
+  await SpecialPowers.spawn(iframeBrowsingContext, [], () => {
+    content.document.querySelector("style").remove();
+  });
+  await waitUntil(() => destroyedResources.length === 4);
+  assertDestroyed(destroyedResources[3], {
+    resourceId: availableResources.find(
+      resource =>
+        findMatchingExpectedResource(resource) === EXISTING_RESOURCES[3]
+    ).resourceId,
+  });
+
+  info("Remove existing iframe <link> stylesheet");
+  await SpecialPowers.spawn(iframeBrowsingContext, [], () => {
+    content.document.querySelector("link").remove();
+  });
+  await waitUntil(() => destroyedResources.length === 5);
+  assertDestroyed(destroyedResources[4], {
+    resourceId: availableResources.find(
+      resource =>
+        findMatchingExpectedResource(resource) === EXISTING_RESOURCES[4]
+    ).resourceId,
+  });
+
+  targetCommand.destroy();
+  await client.close();
+}
+
+async function testResourceUpdateFeature() {
+  info("Check resource update feature of the ResourceCommand");
+
+  const tab = await addTab(STYLE_TEST_URL);
+
+  const { client, resourceCommand, targetCommand } = await initResourceCommand(
+    tab
+  );
+
+  info("Setup the watcher");
+  const availableResources = [];
+  const updates = [];
+  await resourceCommand.watchResources([resourceCommand.TYPES.STYLESHEET], {
+    onAvailable: pushAvailableResource(availableResources),
+    onUpdated: newUpdates => updates.push(...newUpdates),
+  });
+  is(
+    availableResources.length,
+    EXISTING_RESOURCES.length,
+    "Length of existing resources is correct"
+  );
+  is(updates.length, 0, "there's no update yet");
+
+  info("Check toggleDisabled function");
+  // Retrieve the stylesheet of the top-level target
+  const resource = availableResources.find(
+    innerResource => innerResource.targetFront.isTopLevel
+  );
+  const styleSheetsFront = await resource.targetFront.getFront("stylesheets");
+  await styleSheetsFront.toggleDisabled(resource.resourceId);
+  await waitUntil(() => updates.length === 1);
+
+  // Check the content of the update object.
+  assertUpdate(updates[0].update, {
+    resourceId: resource.resourceId,
+    updateType: "property-change",
+  });
+  is(
+    updates[0].update.resourceUpdates.disabled,
+    true,
+    "resourceUpdates is correct"
+  );
+
+  // Check whether the cached resource is updated correctly.
+  is(
+    updates[0].resource.disabled,
+    true,
+    "cached resource is updated correctly"
+  );
+
+  // Check whether the actual stylesheet is updated correctly.
+  const styleSheetDisabled = await ContentTask.spawn(
+    tab.linkedBrowser,
+    null,
+    () => {
+      const document = content.document;
+      const stylesheet = document.styleSheets[0];
+      return stylesheet.disabled;
+    }
+  );
+  is(styleSheetDisabled, true, "actual stylesheet was updated correctly");
+
+  info("Check update function");
+  const expectedAtRules = [
+    {
+      type: "media",
+      conditionText: "screen",
+      matches: true,
+    },
+    {
+      type: "media",
+      conditionText: "print",
+      matches: false,
+    },
+  ];
+
+  const updateCause = "updated-by-test";
+  await styleSheetsFront.update(
+    resource.resourceId,
+    "@media screen { color: red; } @media print { color: green; } body { color: cyan; }",
+    false,
+    updateCause
+  );
+  await waitUntil(() => updates.length === 4);
+
+  assertUpdate(updates[1].update, {
+    resourceId: resource.resourceId,
+    updateType: "property-change",
+  });
+  is(
+    updates[1].update.resourceUpdates.ruleCount,
+    3,
+    "resourceUpdates is correct"
+  );
+  is(updates[1].resource.ruleCount, 3, "cached resource is updated correctly");
+
+  assertUpdate(updates[2].update, {
+    resourceId: resource.resourceId,
+    updateType: "style-applied",
+    event: {
+      cause: updateCause,
+    },
+  });
+  is(
+    updates[2].update.resourceUpdates,
+    undefined,
+    "resourceUpdates is correct"
+  );
+
+  assertUpdate(updates[3].update, {
+    resourceId: resource.resourceId,
+    updateType: "at-rules-changed",
+  });
+  assertAtRules(updates[3].update.resourceUpdates.atRules, expectedAtRules);
+
+  // Check the actual page.
+  const styleSheetResult = await getStyleSheetResult(tab);
+
+  is(
+    styleSheetResult.ruleCount,
+    3,
+    "ruleCount of actual stylesheet is updated correctly"
+  );
+  assertAtRules(styleSheetResult.atRules, expectedAtRules);
+
+  targetCommand.destroy();
+  await client.close();
+}
+
+async function testNestedResourceUpdateFeature() {
+  info("Check nested resource update feature of the ResourceCommand");
+
+  const tab = await addTab(STYLE_TEST_URL);
+
+  const { outerWidth: originalWindowWidth, outerHeight: originalWindowHeight } =
+    tab.ownerGlobal;
+
+  registerCleanupFunction(() => {
+    tab.ownerGlobal.resizeTo(originalWindowWidth, originalWindowHeight);
+  });
+
+  const { client, resourceCommand, targetCommand } = await initResourceCommand(
+    tab
+  );
+
+  info("Setup the watcher");
+  const availableResources = [];
+  const updates = [];
+  await resourceCommand.watchResources([resourceCommand.TYPES.STYLESHEET], {
+    onAvailable: pushAvailableResource(availableResources),
+    onUpdated: newUpdates => updates.push(...newUpdates),
+  });
+  is(
+    availableResources.length,
+    EXISTING_RESOURCES.length,
+    "Length of existing resources is correct"
+  );
+
+  info("Apply new media query");
+  // In order to avoid applying the media query (min-height: 400px).
+  if (originalWindowHeight !== 300) {
+    await new Promise(resolve => {
+      tab.ownerGlobal.addEventListener("resize", resolve, { once: true });
+      tab.ownerGlobal.resizeTo(originalWindowWidth, 300);
+    });
+  }
+
+  // Retrieve the stylesheet of the top-level target
+  const resource = availableResources.find(
+    innerResource => innerResource.targetFront.isTopLevel
+  );
+  const styleSheetsFront = await resource.targetFront.getFront("stylesheets");
+  await styleSheetsFront.update(
+    resource.resourceId,
+    `@media (min-height: 400px) {
+      html {
+        color: red;
+      }
+      @layer myLayer {
+        @supports (container-type) {
+          :root {
+            color: gold;
+            container: root inline-size;
+          }
+
+          @container root (width > 10px) {
+            body {
+              color: gold;
+            }
+          }
+        }
+      }
+    }`,
+    false
+  );
+  await waitUntil(() => updates.length === 3);
+  is(
+    updates.at(-1).resource.ruleCount,
+    7,
+    "Resource in update has expected ruleCount"
+  );
+
+  is(resource.atRules[0].matches, false, "Media query is not matched yet");
+
+  info("Change window size to fire matches-change event");
+  tab.ownerGlobal.resizeTo(originalWindowWidth, 500);
+  await waitUntil(() => updates.length === 4);
+
+  // Check the update content.
+  const targetUpdate = updates[3];
+  assertUpdate(targetUpdate.update, {
+    resourceId: resource.resourceId,
+    updateType: "matches-change",
+  });
+  Assert.strictEqual(
+    resource,
+    targetUpdate.resource,
+    "Update object has the same resource"
+  );
+
+  is(
+    JSON.stringify(targetUpdate.update.nestedResourceUpdates[0].path),
+    JSON.stringify(["atRules", 0, "matches"]),
+    "path of nestedResourceUpdates is correct"
+  );
+  is(
+    targetUpdate.update.nestedResourceUpdates[0].value,
+    true,
+    "value of nestedResourceUpdates is correct"
+  );
+
+  // Check the resource.
+  const expectedAtRules = [
+    {
+      type: "media",
+      conditionText: "(min-height: 400px)",
+      matches: true,
+    },
+    {
+      type: "layer",
+      layerName: "myLayer",
+    },
+    {
+      type: "support",
+      conditionText: "(container-type)",
+    },
+    {
+      type: "container",
+      conditionText: "root (width > 10px)",
+    },
+  ];
+
+  assertAtRules(targetUpdate.resource.atRules, expectedAtRules);
+
+  // Check the actual page.
+  const styleSheetResult = await getStyleSheetResult(tab);
+  is(
+    styleSheetResult.ruleCount,
+    7,
+    "ruleCount of actual stylesheet is updated correctly"
+  );
+  assertAtRules(styleSheetResult.atRules, expectedAtRules);
+
+  tab.ownerGlobal.resizeTo(originalWindowWidth, originalWindowHeight);
+
+  targetCommand.destroy();
+  await client.close();
+}
+
+function findMatchingExpectedResource(resource) {
+  return EXISTING_RESOURCES.find(
+    expected =>
+      resource.href === expected.href &&
+      resource.nodeHref === expected.nodeHref &&
+      resource.ruleCount === expected.ruleCount &&
+      resource.constructed == expected.constructed
+  );
+}
+
+async function getStyleSheetResult(tab) {
+  const result = await ContentTask.spawn(tab.linkedBrowser, null, () => {
+    const document = content.document;
+    const stylesheet = document.styleSheets[0];
+    let ruleCount = 0;
+    const atRules = [];
+
+    const traverseRules = ruleList => {
+      for (const rule of ruleList) {
+        ruleCount++;
+
+        if (rule.media) {
+          let matches = false;
+          try {
+            const mql = content.matchMedia(rule.media.mediaText);
+            matches = mql.matches;
+          } catch (e) {
+            // Ignored
+          }
+
+          atRules.push({
+            type: "media",
+            conditionText: rule.conditionText,
+            matches,
+          });
+        } else if (rule instanceof content.CSSContainerRule) {
+          atRules.push({
+            type: "container",
+            conditionText: rule.conditionText,
+          });
+        } else if (rule instanceof content.CSSLayerBlockRule) {
+          atRules.push({ type: "layer", layerName: rule.name });
+        } else if (rule instanceof content.CSSSupportsRule) {
+          atRules.push({
+            type: "support",
+            conditionText: rule.conditionText,
+          });
+        }
+
+        if (rule.cssRules) {
+          traverseRules(rule.cssRules);
+        }
+      }
+    };
+    traverseRules(stylesheet.cssRules);
+
+    return { ruleCount, atRules };
+  });
+
+  return result;
+}
+
+function assertAtRules(atRules, expectedAtRules) {
+  is(
+    atRules.length,
+    expectedAtRules.length,
+    "Length of the atRules is correct"
+  );
+
+  for (let i = 0; i < atRules.length; i++) {
+    const atRule = atRules[i];
+    const expected = expectedAtRules[i];
+    is(atRule.type, expected.type, "at-rule is of expected type");
+    is(
+      atRules[i].conditionText,
+      expected.conditionText,
+      "conditionText is correct"
+    );
+    if (expected.type === "media") {
+      is(atRule.matches, expected.matches, "matches is correct");
+    } else if (expected.type === "layer") {
+      is(atRule.layerName, expected.layerName, "layerName is correct");
+    }
+
+    if (expected.line !== undefined) {
+      is(atRule.line, expected.line, "line is correct");
+    }
+
+    if (expected.column !== undefined) {
+      is(atRule.column, expected.column, "column is correct");
+    }
+  }
+}
+
+async function assertResource(resource, expected) {
+  is(
+    resource.resourceType,
+    ResourceCommand.TYPES.STYLESHEET,
+    "Resource type is correct"
+  );
+  const styleText = (await getStyleSheetResourceText(resource)).trim();
+  is(styleText, expected.styleText, "Style text is correct");
+  is(resource.href, expected.href, "href is correct");
+  is(resource.nodeHref, expected.nodeHref, "nodeHref is correct");
+  is(resource.isNew, expected.isNew, "isNew is correct");
+  is(resource.disabled, expected.disabled, "disabled is correct");
+  is(resource.constructed, expected.constructed, "constructed is correct");
+  is(resource.ruleCount, expected.ruleCount, "ruleCount is correct");
+  assertAtRules(resource.atRules, expected.atRules);
+}
+
+function assertUpdate(update, expected) {
+  is(
+    update.resourceType,
+    ResourceCommand.TYPES.STYLESHEET,
+    "Resource type is correct"
+  );
+  is(update.resourceId, expected.resourceId, "resourceId is correct");
+  is(update.updateType, expected.updateType, "updateType is correct");
+  if (expected.event?.cause) {
+    is(update.event?.cause, expected.event.cause, "cause is correct");
+  }
+}
+
+function assertDestroyed(resource, expected) {
+  is(
+    resource.resourceType,
+    ResourceCommand.TYPES.STYLESHEET,
+    "Resource type is correct"
+  );
+  is(resource.resourceId, expected.resourceId, "resourceId is correct");
+}
+
+function getResourceTimingCount(tab) {
+  return ContentTask.spawn(tab.linkedBrowser, [], () => {
+    return content.performance.getEntriesByType("resource").length;
+  });
+}
diff --git a/devtools/shared/commands/resource/tests/browser_resources_stylesheets_header.js b/devtools/shared/commands/resource/tests/browser_resources_stylesheets_header.js
new file mode 100644
index 0000000000..29263d887b
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/browser_resources_stylesheets_header.js
@@ -0,0 +1,82 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test that we do get the appropriate stylesheet content when the stylesheet is only
+// served based on the Accept: text/css header
+
+add_task(async function () {
+  const httpServer = createTestHTTPServer();
+
+  httpServer.registerContentType("html", "text/html");
+
+  httpServer.registerPathHandler("/index.html", function (request, response) {
+    response.setStatusLine(request.httpVersion, 200, "OK");
+    response.write(`
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>Test stylesheet</title>
+<link href="/test/" rel="stylesheet" type="text/css"/>
+<script src="/test/"></script>
+<h1>Hello</h1>
+  `);
+  });
+
+  let resourceUrlCalls = 0;
+  // The /test/ URL should be called:
+  // - once by the content page to load the <link>
+  // - once by the content page to load the <script>
+  // - once by DevTools to fetch the stylesheet text
+  // (we could probably optimize this so we only call once)
+  const expectedResourceUrlCalls = 3;
+
+  const styleSheetText = `body { background-color: tomato; }`;
+  httpServer.registerPathHandler("/test/", function (request, response) {
+    resourceUrlCalls++;
+    response.setStatusLine(request.httpVersion, 200, "OK");
+
+    if (request.getHeader("Accept").startsWith("text/css")) {
+      response.setHeader("Content-Type", "text/css", false);
+      response.write(styleSheetText);
+      return;
+    }
+    response.setHeader("Content-Type", "application/javascript", false);
+    response.write(`/* NOT A STYLESHEET */`);
+  });
+  const port = httpServer.identity.primaryPort;
+  const TEST_URL = `http://localhost:${port}/index.html`;
+
+  info("Check resource available feature of the ResourceCommand");
+  const tab = await addTab(TEST_URL);
+
+  const { client, resourceCommand, targetCommand } = await initResourceCommand(
+    tab
+  );
+
+  info("Check whether ResourceCommand gets existing stylesheet");
+  const availableResources = [];
+  await resourceCommand.watchResources([resourceCommand.TYPES.STYLESHEET], {
+    onAvailable: resources => availableResources.push(...resources),
+  });
+  is(
+    availableResources.length,
+    1,
+    "We have the expected number of stylesheets"
+  );
+
+  is(
+    await getStyleSheetResourceText(availableResources[0]),
+    styleSheetText,
+    "Got expected text for the stylesheet"
+  );
+
+  is(
+    resourceUrlCalls,
+    expectedResourceUrlCalls,
+    "The /test URL was called the number of time we expected"
+  );
+
+  targetCommand.destroy();
+  await client.close();
+});
diff --git a/devtools/shared/commands/resource/tests/browser_resources_stylesheets_import.js b/devtools/shared/commands/resource/tests/browser_resources_stylesheets_import.js
new file mode 100644
index 0000000000..c58a5162e0
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/browser_resources_stylesheets_import.js
@@ -0,0 +1,60 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test the ResourceCommand API for imported STYLESHEET + iframe.
+
+const styleSheetText = `
+@import "${URL_ROOT_ORG_SSL}/style_document.css";
+body { background-color: tomato; }`;
+
+const IFRAME_URL = `https://example.org/document-builder.sjs?html=${encodeURIComponent(`
+  <style>${styleSheetText}</style>
+  <h1>iframe</h1>
+`)}`;
+
+const TEST_URL = `https://example.org/document-builder.sjs?html=
+  <h1>import stylesheet test</h1>
+  <iframe src="${encodeURIComponent(IFRAME_URL)}"></iframe>`;
+
+add_task(async function () {
+  info("Check resource available feature of the ResourceCommand");
+
+  const tab = await addTab(TEST_URL);
+
+  const { client, resourceCommand, targetCommand } = await initResourceCommand(
+    tab
+  );
+
+  info("Check whether ResourceCommand gets existing stylesheet");
+  const availableResources = [];
+  await resourceCommand.watchResources([resourceCommand.TYPES.STYLESHEET], {
+    onAvailable: resources => availableResources.push(...resources),
+  });
+
+  await waitFor(() => availableResources.length === 2);
+  ok(true, "We're getting the expected stylesheets");
+
+  const styleNodeStyleSheet = availableResources.find(
+    resource => resource.nodeHref
+  );
+  const importedStyleSheet = availableResources.find(
+    resource => resource !== styleNodeStyleSheet
+  );
+
+  is(
+    await getStyleSheetResourceText(styleNodeStyleSheet),
+    styleSheetText,
+    "Got expected text for the <style> stylesheet"
+  );
+
+  is(
+    (await getStyleSheetResourceText(importedStyleSheet)).trim(),
+    `body { margin: 1px; }`,
+    "Got expected text for the imported stylesheet"
+  );
+
+  targetCommand.destroy();
+  await client.close();
+});
diff --git a/devtools/shared/commands/resource/tests/browser_resources_stylesheets_navigation.js b/devtools/shared/commands/resource/tests/browser_resources_stylesheets_navigation.js
new file mode 100644
index 0000000000..1ee8913bda
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/browser_resources_stylesheets_navigation.js
@@ -0,0 +1,254 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test the ResourceCommand API around STYLESHEET and navigation (reloading, creation of new browsing context, …)
+
+const ORG_DOC_BUILDER = "https://example.org/document-builder.sjs";
+const COM_DOC_BUILDER = "https://example.com/document-builder.sjs";
+
+// Since the order of resources is not guaranteed, we put a number in the title attribute
+// of the <style> elements so we can sort them in a way that makes it easier for us to assert.
+let currentStyleTitle = 0;
+
+const TEST_URI =
+  `${ORG_DOC_BUILDER}?html=1<h1>top-level example.org</h1>` +
+  `<style title="${currentStyleTitle++}">.top-level-org{}</style>` +
+  `<iframe id="same-origin-1" src="${ORG_DOC_BUILDER}?html=<h2>example.org 1</h2><style title=${currentStyleTitle++}>.frame-org-1{}</style>"></iframe>` +
+  `<iframe id="same-origin-2" src="${ORG_DOC_BUILDER}?html=<h2>example.org 2</h2><style title=${currentStyleTitle++}>.frame-org-2{}</style>"></iframe>` +
+  `<iframe id="remote-origin-1" src="${COM_DOC_BUILDER}?html=<h2>example.com 1</h2><style title=${currentStyleTitle++}>.frame-com-1{}</style>"></iframe>` +
+  `<iframe id="remote-origin-2" src="${COM_DOC_BUILDER}?html=<h2>example.com 2</h2><style title=${currentStyleTitle++}>.frame-com-2{}</style>"></iframe>`;
+
+const COOP_HEADERS = "Cross-Origin-Opener-Policy:same-origin";
+const TEST_URI_NEW_BROWSING_CONTEXT =
+  `${ORG_DOC_BUILDER}?headers=${COOP_HEADERS}` +
+  `&html=<h1>top-level example.org</div>` +
+  `<style>.top-level-org-new-bc{}</style>`;
+
+add_task(async function () {
+  info(
+    "Open a new tab and check that styleSheetChangeEventsEnabled is false by default"
+  );
+  const tab = await addTab(TEST_URI);
+
+  is(
+    await getDocumentStyleSheetChangeEventsEnabled(tab.linkedBrowser),
+    false,
+    `styleSheetChangeEventsEnabled is false at the beginning`
+  );
+
+  const { client, resourceCommand, targetCommand } = await initResourceCommand(
+    tab
+  );
+
+  let availableResources = [];
+  await resourceCommand.watchResources([resourceCommand.TYPES.STYLESHEET], {
+    onAvailable: resources => {
+      availableResources.push(...resources);
+    },
+  });
+
+  info("Wait for all the stylesheets resources of main document and iframes");
+  await waitFor(() => availableResources.length === 5);
+  is(availableResources.length, 5, "Retrieved the expected stylesheets");
+
+  // the order of the resources is not guaranteed.
+  sortResourcesByExpectedOrder(availableResources);
+  await assertResource(availableResources[0], {
+    styleText: `.top-level-org{}`,
+  });
+  await assertResource(availableResources[1], {
+    styleText: `.frame-org-1{}`,
+  });
+  await assertResource(availableResources[2], {
+    styleText: `.frame-org-2{}`,
+  });
+  await assertResource(availableResources[3], {
+    styleText: `.frame-com-1{}`,
+  });
+  await assertResource(availableResources[4], {
+    styleText: `.frame-com-2{}`,
+  });
+
+  // clear availableResources so it's easier to test
+  availableResources = [];
+
+  is(
+    await getDocumentStyleSheetChangeEventsEnabled(tab.linkedBrowser),
+    true,
+    `styleSheetChangeEventsEnabled is true after watching stylesheets`
+  );
+
+  info("Navigate a remote frame to a different page");
+  const iframeNewUrl =
+    `https://example.com/document-builder.sjs?` +
+    `html=<h2>example.com new bc</h2><style title=6>.frame-com-new-bc{}</style>`;
+  await SpecialPowers.spawn(tab.linkedBrowser, [iframeNewUrl], url => {
+    const { browsingContext } =
+      content.document.querySelector("#remote-origin-2");
+    return SpecialPowers.spawn(browsingContext, [url], innerUrl => {
+      content.document.location = innerUrl;
+    });
+  });
+  await waitFor(() => availableResources.length == 1);
+  ok(true, "We're notified about the iframe new document stylesheet");
+  await assertResource(availableResources[0], {
+    styleText: `.frame-com-new-bc{}`,
+  });
+  const iframeNewBrowsingContext = await SpecialPowers.spawn(
+    tab.linkedBrowser,
+    [],
+    () => content.document.querySelector("#remote-origin-2").browsingContext
+  );
+
+  is(
+    await getDocumentStyleSheetChangeEventsEnabled(iframeNewBrowsingContext),
+    true,
+    `styleSheetChangeEventsEnabled is still true after navigating the iframe`
+  );
+
+  // clear availableResources so it's easier to test
+  availableResources = [];
+
+  info("Check that styleSheetChangeEventsEnabled persist after reloading");
+  await reloadBrowser();
+
+  // ⚠️ When EFT is disabled, we're only getting the stylesheets for the top-level document
+  // and the remote frames; the same-origin iframes stylesheets are missing.
+  const expectedStylesheetResources = isEveryFrameTargetEnabled() ? 5 : 3;
+  info(
+    "Wait until we're notified about all the stylesheets (top-level document + iframe)"
+  );
+  await waitFor(
+    () => availableResources.length === expectedStylesheetResources
+  );
+  is(
+    availableResources.length,
+    expectedStylesheetResources,
+    "Retrieved the expected stylesheets after the page was reloaded"
+  );
+
+  // the order of the resources is not guaranteed.
+  sortResourcesByExpectedOrder(availableResources);
+  await assertResource(availableResources[0], {
+    styleText: `.top-level-org{}`,
+  });
+  if (isEveryFrameTargetEnabled()) {
+    await assertResource(availableResources[1], {
+      styleText: `.frame-org-1{}`,
+    });
+    await assertResource(availableResources[2], {
+      styleText: `.frame-org-2{}`,
+    });
+    await assertResource(availableResources[3], {
+      styleText: `.frame-com-1{}`,
+    });
+    await assertResource(availableResources[4], {
+      styleText: `.frame-com-new-bc{}`,
+    });
+  } else {
+    await assertResource(availableResources[1], {
+      styleText: `.frame-com-1{}`,
+    });
+    await assertResource(availableResources[2], {
+      styleText: `.frame-com-new-bc{}`,
+    });
+  }
+
+  is(
+    await getDocumentStyleSheetChangeEventsEnabled(tab.linkedBrowser),
+    true,
+    `styleSheetChangeEventsEnabled is still true on the top level document after reloading`
+  );
+
+  if (isEveryFrameTargetEnabled()) {
+    const bc = await SpecialPowers.spawn(
+      tab.linkedBrowser,
+      [],
+      () => content.document.querySelector("#same-origin-1").browsingContext
+    );
+    is(
+      await getDocumentStyleSheetChangeEventsEnabled(bc),
+      true,
+      `styleSheetChangeEventsEnabled is still true on the iframe after reloading`
+    );
+  }
+
+  // clear availableResources so it's easier to test
+  availableResources = [];
+
+  info(
+    "Check that styleSheetChangeEventsEnabled persist when navigating to a page that creates a new browsing context"
+  );
+  const previousBrowsingContextId = tab.linkedBrowser.browsingContext.id;
+  const onLoaded = BrowserTestUtils.browserLoaded(tab.linkedBrowser);
+  BrowserTestUtils.startLoadingURIString(
+    tab.linkedBrowser,
+    TEST_URI_NEW_BROWSING_CONTEXT
+  );
+  await onLoaded;
+
+  isnot(
+    tab.linkedBrowser.browsingContext.id,
+    previousBrowsingContextId,
+    "A new browsing context was created"
+  );
+
+  info("Wait to get the stylesheet for the new document");
+  await waitFor(() => availableResources.length === 1);
+  ok(true, "We received the stylesheet for the new document");
+  await assertResource(availableResources[0], {
+    styleText: `.top-level-org-new-bc{}`,
+  });
+  is(
+    await getDocumentStyleSheetChangeEventsEnabled(tab.linkedBrowser),
+    true,
+    `styleSheetChangeEventsEnabled is still true after navigating to a new browsing context`
+  );
+
+  targetCommand.destroy();
+  await client.close();
+});
+
+/**
+ * Returns the value of the browser/browsingContext document `styleSheetChangeEventsEnabled`
+ * property.
+ *
+ * @param {Browser|BrowsingContext} browserOrBrowsingContext: The browser element or a
+ *        browsing context.
+ * @returns {Promise<Boolean>}
+ */
+function getDocumentStyleSheetChangeEventsEnabled(browserOrBrowsingContext) {
+  return SpecialPowers.spawn(browserOrBrowsingContext, [], () => {
+    return content.document.styleSheetChangeEventsEnabled;
+  });
+}
+
+/**
+ * Sort the passed array of stylesheet resources.
+ *
+ * Since the order of resources are not guaranteed, the <style> elements we use in this test
+ * have a "title" attribute that represent their expected order so we can sort them in
+ * a way that makes it easier for us to assert.
+ *
+ * @param {Array<Object>} resources: Array of stylesheet resources
+ */
+function sortResourcesByExpectedOrder(resources) {
+  resources.sort((a, b) => {
+    return Number(a.title) > Number(b.title);
+  });
+}
+
+/**
+ * Check that the resources have the expected text
+ *
+ * @param {Array<Object>} resources: Array of stylesheet resources
+ * @param {Array<Object>} expected: Array of object of the following shape:
+ * @param {Object} expected[]
+ * @param {Object} expected[].styleText: Expected text content of the stylesheet
+ */
+async function assertResource(resource, expected) {
+  const styleText = (await getStyleSheetResourceText(resource)).trim();
+  is(styleText, expected.styleText, "Style text is correct");
+}
diff --git a/devtools/shared/commands/resource/tests/browser_resources_stylesheets_nested_iframes.js b/devtools/shared/commands/resource/tests/browser_resources_stylesheets_nested_iframes.js
new file mode 100644
index 0000000000..0b13f75ab9
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/browser_resources_stylesheets_nested_iframes.js
@@ -0,0 +1,34 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test that stylesheets are retrieved even if an iframe does not have a content document.
+
+const TEST_URI = URL_ROOT_SSL + "stylesheets-nested-iframes.html";
+
+add_task(async function () {
+  const tab = await addTab(TEST_URI);
+
+  const { client, resourceCommand, targetCommand } = await initResourceCommand(
+    tab
+  );
+
+  info("Check whether ResourceCommand gets existing stylesheet");
+  const availableResources = [];
+  await resourceCommand.watchResources([resourceCommand.TYPES.STYLESHEET], {
+    onAvailable: resources => availableResources.push(...resources),
+  });
+
+  // Bug 285395 limits the number of nested iframes to 10, and we have one stylesheet per document.
+  await waitFor(() => availableResources.length >= 10);
+
+  is(
+    availableResources.length,
+    10,
+    "Got the expected number of stylesheets, even with documentless iframes"
+  );
+
+  targetCommand.destroy();
+  await client.close();
+});
diff --git a/devtools/shared/commands/resource/tests/browser_resources_target_destroy.js b/devtools/shared/commands/resource/tests/browser_resources_target_destroy.js
new file mode 100644
index 0000000000..fa7813d26e
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/browser_resources_target_destroy.js
@@ -0,0 +1,104 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test that the server ResourceCommand are destroyed when the associated target actors
+// are destroyed.
+
+add_task(async function () {
+  const tab = await addTab("data:text/html,Test");
+  const { client, resourceCommand, targetCommand } = await initResourceCommand(
+    tab
+  );
+
+  // Start watching for console messages. We don't care about messages here, only the
+  // registration/destroy mechanism, so we make onAvailable a no-op function.
+  await resourceCommand.watchResources(
+    [resourceCommand.TYPES.CONSOLE_MESSAGE],
+    {
+      onAvailable: () => {},
+    }
+  );
+
+  info(
+    "Spawn a content task in order to be able to manipulate actors and resource watchers directly"
+  );
+  const connectionPrefix = targetCommand.watcherFront.actorID.replace(
+    /watcher\d+$/,
+    ""
+  );
+  await ContentTask.spawn(
+    tab.linkedBrowser,
+    [connectionPrefix],
+    function (_connectionPrefix) {
+      const { require } = ChromeUtils.importESModule(
+        "resource://devtools/shared/loader/Loader.sys.mjs"
+      );
+      const { TargetActorRegistry } = ChromeUtils.importESModule(
+        "resource://devtools/server/actors/targets/target-actor-registry.sys.mjs"
+      );
+      const {
+        getResourceWatcher,
+        TYPES,
+      } = require("resource://devtools/server/actors/resources/index.js");
+
+      // Retrieve the target actor instance and its watcher for console messages
+      const targetActor = TargetActorRegistry.getTargetActors(
+        {
+          type: "browser-element",
+          browserId: content.browsingContext.browserId,
+        },
+        _connectionPrefix
+      ).find(actor => actor.isTopLevelTarget);
+      ok(
+        targetActor,
+        "Got the top level target actor from the content process"
+      );
+      const watcher = getResourceWatcher(targetActor, TYPES.CONSOLE_MESSAGE);
+
+      // Storing the target actor in the global so we can retrieve it later, even if it
+      // was destroyed
+      content._testTargetActor = targetActor;
+
+      is(!!watcher, true, "The console message resource watcher was created");
+    }
+  );
+
+  info("Close the client, which will destroy the target");
+  targetCommand.destroy();
+  await client.close();
+
+  info(
+    "Spawn a content task in order to run some assertions on actors and resource watchers directly"
+  );
+  await ContentTask.spawn(tab.linkedBrowser, [], function () {
+    const { require } = ChromeUtils.importESModule(
+      "resource://devtools/shared/loader/Loader.sys.mjs"
+    );
+    const {
+      getResourceWatcher,
+      TYPES,
+    } = require("resource://devtools/server/actors/resources/index.js");
+
+    ok(
+      content._testTargetActor && !content._testTargetActor.actorID,
+      "The target was destroyed when the client was closed"
+    );
+
+    // Retrieve the console message resource watcher
+    const watcher = getResourceWatcher(
+      content._testTargetActor,
+      TYPES.CONSOLE_MESSAGE
+    );
+
+    is(
+      !!watcher,
+      false,
+      "The console message resource watcher isn't registered anymore after the target was destroyed"
+    );
+
+    // Cleanup work variable
+    delete content._testTargetActor;
+  });
+});
diff --git a/devtools/shared/commands/resource/tests/browser_resources_target_resources_race.js b/devtools/shared/commands/resource/tests/browser_resources_target_resources_race.js
new file mode 100644
index 0000000000..557d14380a
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/browser_resources_target_resources_race.js
@@ -0,0 +1,70 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+/**
+ * Test initial target resources are correctly retrieved even when several calls
+ * to watchResources are made simultaneously.
+ *
+ * This checks a race condition which occurred when calling watchResources
+ * simultaneously. This made the "second" call to watchResources miss existing
+ * resources (in case those are emitted from the target instead of the watcher).
+ * See Bug 1663896.
+ */
+add_task(async function () {
+  // Disable the preloaded process as it creates processes intermittently
+  // which forces the emission of RDP requests we aren't correctly waiting for.
+  await pushPref("dom.ipc.processPrelaunch.enabled", false);
+
+  const { client, resourceCommand, targetCommand } =
+    await initMultiProcessResourceCommand();
+
+  const expectedPlatformMessage = "expectedMessage";
+
+  info("Log a message *before* calling ResourceCommand.watchResources");
+  Services.console.logStringMessage(expectedPlatformMessage);
+
+  info("Call watchResources from 2 separate call sites consecutively");
+
+  // Empty onAvailable callback for CSS MESSAGES, we only want to check that
+  // the second resource we watch correctly provides existing resources.
+  const onCssMessageAvailable = resources => {};
+
+  // First call to watchResources.
+  // We do not await on `watchPromise1` here, in order to simulate simultaneous
+  // calls to watchResources (which could come from 2 separate modules in a real
+  // scenario).
+  const initialWatchPromise = resourceCommand.watchResources(
+    [resourceCommand.TYPES.CSS_MESSAGE],
+    {
+      onAvailable: onCssMessageAvailable,
+    }
+  );
+
+  // `waitForNextResource` will trigger another call to `watchResources`.
+  const { onResource: onMessageReceived } =
+    await resourceCommand.waitForNextResource(
+      resourceCommand.TYPES.PLATFORM_MESSAGE,
+      {
+        ignoreExistingResources: false,
+        predicate: r => r.message === expectedPlatformMessage,
+      }
+    );
+
+  info("Waiting for the expected message to be received");
+  await onMessageReceived;
+  ok(true, "All the expected messages were received");
+
+  info("Wait for the other watchResources promise to finish");
+  await initialWatchPromise;
+
+  // Unwatch all resources.
+  resourceCommand.unwatchResources([resourceCommand.TYPES.CSS_MESSAGE], {
+    onAvailable: onCssMessageAvailable,
+  });
+
+  Services.console.reset();
+  targetCommand.destroy();
+  await client.close();
+});
diff --git a/devtools/shared/commands/resource/tests/browser_resources_target_switching.js b/devtools/shared/commands/resource/tests/browser_resources_target_switching.js
new file mode 100644
index 0000000000..4551fec778
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/browser_resources_target_switching.js
@@ -0,0 +1,94 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test the behavior of ResourceCommand when the top level target changes
+
+const TEST_URI =
+  "data:text/html;charset=utf-8,<script>console.log('foo');</script>";
+
+add_task(async function () {
+  const tab = await addTab(TEST_URI);
+
+  const { client, resourceCommand, targetCommand } = await initResourceCommand(
+    tab
+  );
+  const { CONSOLE_MESSAGE, SOURCE } = resourceCommand.TYPES;
+
+  info("Check the resources gotten from getAllResources at initial");
+  is(
+    resourceCommand.getAllResources(CONSOLE_MESSAGE).length,
+    0,
+    "There is no resources before calling watchResources"
+  );
+
+  info(
+    "Start to watch the available resources in order to compare with resources gotten from getAllResources"
+  );
+  const availableResources = [];
+  const onAvailable = resources => {
+    availableResources.push(...resources);
+  };
+  await resourceCommand.watchResources([CONSOLE_MESSAGE], { onAvailable });
+
+  is(availableResources.length, 1, "Got the page message");
+  is(
+    availableResources[0].message.arguments[0],
+    "foo",
+    "Got the expected page message"
+  );
+
+  // Register another listener before unregistering the console listener
+  // otherwise the resource command stop watching for targets
+  const onSourceAvailable = () => {};
+  await resourceCommand.watchResources([SOURCE], {
+    onAvailable: onSourceAvailable,
+  });
+
+  info(
+    "Unregister the console listener and check that we no longer listen for console messages"
+  );
+  resourceCommand.unwatchResources([CONSOLE_MESSAGE], {
+    onAvailable,
+  });
+
+  let onSwitched = targetCommand.once("switched-target");
+  info("Navigate to another process");
+  BrowserTestUtils.startLoadingURIString(
+    gBrowser.selectedBrowser,
+    "about:robots"
+  );
+  await BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser);
+  await onSwitched;
+
+  is(
+    availableResources.length,
+    1,
+    "about:robots doesn't fire any new message, so we should have a new one"
+  );
+
+  info("Navigate back to data: URI");
+  onSwitched = targetCommand.once("switched-target");
+  BrowserTestUtils.startLoadingURIString(gBrowser.selectedBrowser, TEST_URI);
+  await BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser);
+  await onSwitched;
+
+  is(
+    availableResources.length,
+    1,
+    "the data:URI fired a message, but we are no longer listening to it, so no new one should be notified"
+  );
+  is(
+    resourceCommand.getAllResources(CONSOLE_MESSAGE).length,
+    0,
+    "As we are no longer listening to CONSOLE message, we should not collect any"
+  );
+
+  resourceCommand.unwatchResources([SOURCE], {
+    onAvailable: onSourceAvailable,
+  });
+
+  targetCommand.destroy();
+  await client.close();
+});
diff --git a/devtools/shared/commands/resource/tests/browser_resources_thread_states.js b/devtools/shared/commands/resource/tests/browser_resources_thread_states.js
new file mode 100644
index 0000000000..f915bb14d0
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/browser_resources_thread_states.js
@@ -0,0 +1,557 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test the ResourceCommand API around THREAD_STATE
+
+const ResourceCommand = require("resource://devtools/shared/commands/resource/resource-command.js");
+
+const BREAKPOINT_TEST_URL = URL_ROOT_SSL + "breakpoint_document.html";
+const REMOTE_IFRAME_URL =
+  "https://example.org/document-builder.sjs?html=" +
+  encodeURIComponent("<script>debugger;</script>");
+
+add_task(async function () {
+  // Check hitting the "debugger;" statement before and after calling
+  // watchResource(THREAD_TYPES). Both should break. First will
+  // be a cached resource and second will be a live one.
+  await checkBreakpointBeforeWatchResources();
+  await checkBreakpointAfterWatchResources();
+
+  // Check setting a real breakpoint on a given line
+  await checkRealBreakpoint();
+
+  // Check the "pause on exception" setting
+  await checkPauseOnException();
+
+  // Check an edge case where spamming setBreakpoints calls causes issues
+  await checkSetBeforeWatch();
+
+  // Check debugger statement for (remote) iframes
+  await checkDebuggerStatementInIframes();
+});
+
+async function checkBreakpointBeforeWatchResources() {
+  info(
+    "Check whether ResourceCommand gets existing breakpoint, being hit before calling watchResources"
+  );
+
+  const tab = await addTab(BREAKPOINT_TEST_URL);
+
+  const { client, resourceCommand, targetCommand } = await initResourceCommand(
+    tab
+  );
+
+  // Ensure that the target front is initialized early from TargetCommand.onTargetAvailable
+  // By the time `initResourceCommand` resolves, it should already be initialized.
+  info(
+    "Verify that TargetFront's initialized is resolved after having calling attachAndInitThread"
+  );
+  await targetCommand.targetFront.initialized;
+
+  info("Run the 'debugger' statement");
+  // Note that we do not wait for the resolution of spawn as it will be paused
+  ContentTask.spawn(tab.linkedBrowser, null, () => {
+    content.window.wrappedJSObject.runDebuggerStatement();
+  });
+
+  info("Call watchResources");
+  const availableResources = [];
+  await resourceCommand.watchResources([resourceCommand.TYPES.THREAD_STATE], {
+    onAvailable: resources => availableResources.push(...resources),
+  });
+
+  is(
+    availableResources.length,
+    1,
+    "Got the THREAD_STATE's related to the debugger statement"
+  );
+  const threadState = availableResources.pop();
+
+  assertPausedResource(threadState, {
+    state: "paused",
+    why: {
+      type: "debuggerStatement",
+    },
+    frame: {
+      type: "call",
+      asyncCause: null,
+      state: "on-stack",
+      // this: object actor's form referring to `this` variable
+      displayName: "runDebuggerStatement",
+      // arguments: []
+      where: {
+        line: 17,
+        column: 6,
+      },
+    },
+  });
+
+  const { threadFront } = targetCommand.targetFront;
+  await threadFront.resume();
+
+  await waitFor(
+    () => availableResources.length == 1,
+    "Wait until we receive the resumed event"
+  );
+
+  const resumed = availableResources.pop();
+
+  assertResumedResource(resumed);
+
+  targetCommand.destroy();
+  await client.close();
+}
+
+async function checkBreakpointAfterWatchResources() {
+  info(
+    "Check whether ResourceCommand gets breakpoint hit after calling watchResources"
+  );
+
+  const tab = await addTab(BREAKPOINT_TEST_URL);
+
+  const { client, resourceCommand, targetCommand } = await initResourceCommand(
+    tab
+  );
+
+  info("Call watchResources");
+  const availableResources = [];
+  await resourceCommand.watchResources([resourceCommand.TYPES.THREAD_STATE], {
+    onAvailable: resources => availableResources.push(...resources),
+  });
+
+  is(
+    availableResources.length,
+    0,
+    "Got no THREAD_STATE when calling watchResources"
+  );
+
+  info("Run the 'debugger' statement");
+  // Note that we do not wait for the resolution of spawn as it will be paused
+  ContentTask.spawn(tab.linkedBrowser, null, () => {
+    content.window.wrappedJSObject.runDebuggerStatement();
+  });
+
+  await waitFor(
+    () => availableResources.length == 1,
+    "Got the THREAD_STATE related to the debugger statement"
+  );
+  const threadState = availableResources.pop();
+
+  assertPausedResource(threadState, {
+    state: "paused",
+    why: {
+      type: "debuggerStatement",
+    },
+    frame: {
+      type: "call",
+      asyncCause: null,
+      state: "on-stack",
+      // this: object actor's form referring to `this` variable
+      displayName: "runDebuggerStatement",
+      // arguments: []
+      where: {
+        line: 17,
+        column: 6,
+      },
+    },
+  });
+
+  // treadFront is created and attached while calling watchResources
+  const { threadFront } = targetCommand.targetFront;
+
+  await threadFront.resume();
+
+  await waitFor(
+    () => availableResources.length == 1,
+    "Wait until we receive the resumed event"
+  );
+
+  const resumed = availableResources.pop();
+
+  assertResumedResource(resumed);
+
+  targetCommand.destroy();
+  await client.close();
+}
+
+async function checkRealBreakpoint() {
+  info(
+    "Check whether ResourceCommand gets breakpoint set via the thread Front (instead of just debugger statements)"
+  );
+
+  const tab = await addTab(BREAKPOINT_TEST_URL);
+
+  const { client, resourceCommand, targetCommand } = await initResourceCommand(
+    tab
+  );
+
+  info("Call watchResources");
+  const availableResources = [];
+  await resourceCommand.watchResources([resourceCommand.TYPES.THREAD_STATE], {
+    onAvailable: resources => availableResources.push(...resources),
+  });
+
+  is(
+    availableResources.length,
+    0,
+    "Got no THREAD_STATE when calling watchResources"
+  );
+
+  // treadFront is created and attached while calling watchResources
+  const { threadFront } = targetCommand.targetFront;
+
+  // We have to call `sources` request, otherwise the Thread Actor
+  // doesn't start watching for sources, and ignore the setBreakpoint call
+  // as it doesn't have any source registered.
+  await threadFront.getSources();
+
+  await threadFront.setBreakpoint(
+    { sourceUrl: BREAKPOINT_TEST_URL, line: 14 },
+    {}
+  );
+
+  info("Run the test function where we set a breakpoint");
+  // Note that we do not wait for the resolution of spawn as it will be paused
+  ContentTask.spawn(tab.linkedBrowser, null, () => {
+    content.window.wrappedJSObject.testFunction();
+  });
+
+  await waitFor(
+    () => availableResources.length == 1,
+    "Got the THREAD_STATE related to the debugger statement"
+  );
+  const threadState = availableResources.pop();
+
+  assertPausedResource(threadState, {
+    state: "paused",
+    why: {
+      type: "breakpoint",
+    },
+    frame: {
+      type: "call",
+      asyncCause: null,
+      state: "on-stack",
+      // this: object actor's form referring to `this` variable
+      displayName: "testFunction",
+      // arguments: []
+      where: {
+        line: 14,
+        column: 6,
+      },
+    },
+  });
+
+  await threadFront.resume();
+
+  await waitFor(
+    () => availableResources.length == 1,
+    "Wait until we receive the resumed event"
+  );
+
+  const resumed = availableResources.pop();
+
+  assertResumedResource(resumed);
+
+  targetCommand.destroy();
+  await client.close();
+}
+
+async function checkPauseOnException() {
+  info(
+    "Check whether ResourceCommand gets breakpoint for exception (when explicitly requested)"
+  );
+
+  const tab = await addTab(
+    "data:text/html,<meta charset=utf8><script>a.b.c.d</script>"
+  );
+
+  const { commands, resourceCommand, targetCommand } =
+    await initResourceCommand(tab);
+
+  info("Call watchResources");
+  const availableResources = [];
+  await resourceCommand.watchResources([resourceCommand.TYPES.THREAD_STATE], {
+    onAvailable: resources => availableResources.push(...resources),
+  });
+
+  is(
+    availableResources.length,
+    0,
+    "Got no THREAD_STATE when calling watchResources"
+  );
+
+  await commands.threadConfigurationCommand.updateConfiguration({
+    pauseOnExceptions: true,
+  });
+
+  info("Reload the page, in order to trigger exception on load");
+  const reloaded = reloadBrowser();
+
+  await waitFor(
+    () => availableResources.length == 1,
+    "Got the THREAD_STATE related to the debugger statement"
+  );
+  const threadState = availableResources.pop();
+
+  assertPausedResource(threadState, {
+    state: "paused",
+    why: {
+      type: "exception",
+    },
+    frame: {
+      type: "global",
+      asyncCause: null,
+      state: "on-stack",
+      // this: object actor's form referring to `this` variable
+      displayName: "(global)",
+      // arguments: []
+      where: {
+        line: 1,
+        column: 27,
+      },
+    },
+  });
+
+  const { threadFront } = targetCommand.targetFront;
+  await threadFront.resume();
+  info("Wait for page to finish reloading after resume");
+  await reloaded;
+
+  await waitFor(
+    () => availableResources.length == 1,
+    "Wait until we receive the resumed event"
+  );
+
+  const resumed = availableResources.pop();
+
+  assertResumedResource(resumed);
+
+  targetCommand.destroy();
+  await commands.destroy();
+}
+
+async function checkSetBeforeWatch() {
+  info(
+    "Verify bug 1683139 - D103068, where setting a breakpoint before watching for thread state, avoid receiving the paused state"
+  );
+
+  const tab = await addTab(BREAKPOINT_TEST_URL);
+
+  const { client, resourceCommand, targetCommand } = await initResourceCommand(
+    tab
+  );
+
+  // Instantiate the thread front in order to be able to set a breakpoint before watching for thread state
+  info("Attach the top level thread actor");
+  await targetCommand.targetFront.attachAndInitThread(targetCommand);
+  const { threadFront } = targetCommand.targetFront;
+
+  // We have to call `sources` request, otherwise the Thread Actor
+  // doesn't start watching for sources, and ignore the setBreakpoint call
+  // as it doesn't have any source registered.
+  await threadFront.getSources();
+
+  // Set the breakpoint before trying to hit it
+  await threadFront.setBreakpoint(
+    { sourceUrl: BREAKPOINT_TEST_URL, line: 14, column: 6 },
+    {}
+  );
+
+  info("Run the test function where we set a breakpoint");
+  // Note that we do not wait for the resolution of spawn as it will be paused
+  ContentTask.spawn(tab.linkedBrowser, null, () => {
+    content.window.wrappedJSObject.testFunction();
+  });
+
+  // bug 1683139 - D103068. Re-setting the breakpoint just before watching for thread state
+  // prevented to receive the paused state change.
+  await threadFront.setBreakpoint(
+    { sourceUrl: BREAKPOINT_TEST_URL, line: 14, column: 6 },
+    {}
+  );
+
+  info("Call watchResources");
+  const availableResources = [];
+  await resourceCommand.watchResources([resourceCommand.TYPES.THREAD_STATE], {
+    onAvailable: resources => availableResources.push(...resources),
+  });
+
+  await waitFor(
+    () => availableResources.length == 1,
+    "Got the THREAD_STATE related to the debugger statement"
+  );
+  const threadState = availableResources.pop();
+
+  assertPausedResource(threadState, {
+    state: "paused",
+    why: {
+      type: "breakpoint",
+    },
+    frame: {
+      type: "call",
+      asyncCause: null,
+      state: "on-stack",
+      // this: object actor's form referring to `this` variable
+      displayName: "testFunction",
+      // arguments: []
+      where: {
+        line: 14,
+        column: 6,
+      },
+    },
+  });
+
+  await threadFront.resume();
+
+  await waitFor(
+    () => availableResources.length == 1,
+    "Wait until we receive the resumed event"
+  );
+
+  const resumed = availableResources.pop();
+
+  assertResumedResource(resumed);
+
+  targetCommand.destroy();
+  await client.close();
+}
+
+async function checkDebuggerStatementInIframes() {
+  info("Check whether ResourceCommand gets breakpoint for (remote) iframes");
+
+  const tab = await addTab(BREAKPOINT_TEST_URL);
+
+  const { client, resourceCommand, targetCommand } = await initResourceCommand(
+    tab
+  );
+
+  info("Call watchResources");
+  const availableResources = [];
+  await resourceCommand.watchResources([resourceCommand.TYPES.THREAD_STATE], {
+    onAvailable: resources => availableResources.push(...resources),
+  });
+
+  is(
+    availableResources.length,
+    0,
+    "Got no THREAD_STATE when calling watchResources"
+  );
+
+  info("Inject the iframe with an inline 'debugger' statement");
+  // Note that we do not wait for the resolution of spawn as it will be paused
+  SpecialPowers.spawn(
+    gBrowser.selectedBrowser,
+    [REMOTE_IFRAME_URL],
+    async function (url) {
+      const iframe = content.document.createElement("iframe");
+      iframe.src = url;
+      content.document.body.appendChild(iframe);
+    }
+  );
+
+  await waitFor(
+    () => availableResources.length == 1,
+    "Got the THREAD_STATE related to the iframe's debugger statement"
+  );
+  const threadState = availableResources.pop();
+
+  assertPausedResource(threadState, {
+    state: "paused",
+    why: {
+      type: "debuggerStatement",
+    },
+    frame: {
+      type: "global",
+      asyncCause: null,
+      state: "on-stack",
+      // this: object actor's form referring to `this` variable
+      displayName: "(global)",
+      // arguments: []
+      where: {
+        line: 1,
+        column: 8,
+      },
+    },
+  });
+
+  const iframeTarget = threadState.targetFront;
+  if (isFissionEnabled() || isEveryFrameTargetEnabled()) {
+    is(
+      iframeTarget.url,
+      REMOTE_IFRAME_URL,
+      "With fission/EFT, the pause is from the iframe's target"
+    );
+  } else {
+    is(
+      iframeTarget,
+      targetCommand.targetFront,
+      "Without fission/EFT, the pause is from the top level target"
+    );
+  }
+  const { threadFront } = iframeTarget;
+
+  await threadFront.resume();
+
+  await waitFor(
+    () => availableResources.length == 1,
+    "Wait until we receive the resumed event"
+  );
+
+  const resumed = availableResources.pop();
+
+  assertResumedResource(resumed);
+
+  targetCommand.destroy();
+  await client.close();
+}
+
+async function assertPausedResource(resource, expected) {
+  is(
+    resource.resourceType,
+    ResourceCommand.TYPES.THREAD_STATE,
+    "Resource type is correct"
+  );
+  is(resource.state, "paused", "state attribute is correct");
+  is(resource.why.type, expected.why.type, "why.type attribute is correct");
+  is(
+    resource.frame.type,
+    expected.frame.type,
+    "frame.type attribute is correct"
+  );
+  is(
+    resource.frame.asyncCause,
+    expected.frame.asyncCause,
+    "frame.asyncCause attribute is correct"
+  );
+  is(
+    resource.frame.state,
+    expected.frame.state,
+    "frame.state attribute is correct"
+  );
+  is(
+    resource.frame.displayName,
+    expected.frame.displayName,
+    "frame.displayName attribute is correct"
+  );
+  is(
+    resource.frame.where.line,
+    expected.frame.where.line,
+    "frame.where.line attribute is correct"
+  );
+  is(
+    resource.frame.where.column,
+    expected.frame.where.column,
+    "frame.where.column attribute is correct"
+  );
+}
+
+async function assertResumedResource(resource) {
+  is(
+    resource.resourceType,
+    ResourceCommand.TYPES.THREAD_STATE,
+    "Resource type is correct"
+  );
+  is(resource.state, "resumed", "state attribute is correct");
+}
diff --git a/devtools/shared/commands/resource/tests/browser_resources_unwatch_early.js b/devtools/shared/commands/resource/tests/browser_resources_unwatch_early.js
new file mode 100644
index 0000000000..e3890cf970
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/browser_resources_unwatch_early.js
@@ -0,0 +1,113 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test that calling unwatchResources before watchResources could resolve still
+// removes watcher entries correctly.
+
+const ResourceCommand = require("resource://devtools/shared/commands/resource/resource-command.js");
+
+const TEST_URI = "data:text/html;charset=utf-8,";
+
+add_task(async function () {
+  const tab = await addTab(TEST_URI);
+
+  const { client, resourceCommand, targetCommand } = await initResourceCommand(
+    tab
+  );
+  const { CONSOLE_MESSAGE, ROOT_NODE } = resourceCommand.TYPES;
+
+  info("Use console.log in the content page");
+  await logInTab(tab, "msg-1");
+
+  info("Call watchResources with various configurations");
+
+  // Watcher 1 only watches for CONSOLE_MESSAGE.
+  // For this call site, unwatchResource will be called before onAvailable has
+  // resolved.
+  const messages1 = [];
+  const onAvailable1 = createMessageCallback(messages1);
+  const onWatcher1Ready = resourceCommand.watchResources([CONSOLE_MESSAGE], {
+    onAvailable: onAvailable1,
+  });
+  resourceCommand.unwatchResources([CONSOLE_MESSAGE], {
+    onAvailable: onAvailable1,
+  });
+
+  info(
+    "Calling unwatchResources for an already unregistered callback should be a no-op"
+  );
+  // and more importantly, it should not throw
+  resourceCommand.unwatchResources([CONSOLE_MESSAGE], {
+    onAvailable: onAvailable1,
+  });
+
+  // Watcher 2 watches for CONSOLE_MESSAGE & another resource (ROOT_NODE).
+  // Again unwatchResource will be called before onAvailable has resolved.
+  // But unwatchResource is only called for CONSOLE_MESSAGE, not for ROOT_NODE.
+  const messages2 = [];
+  const onAvailable2 = createMessageCallback(messages2);
+  const onWatcher2Ready = resourceCommand.watchResources(
+    [CONSOLE_MESSAGE, ROOT_NODE],
+    {
+      onAvailable: onAvailable2,
+    }
+  );
+  resourceCommand.unwatchResources([CONSOLE_MESSAGE], {
+    onAvailable: onAvailable2,
+  });
+
+  // Watcher 3 watches for CONSOLE_MESSAGE, but we will not call unwatchResource
+  // explicitly for it before the end of test. Used as a reference.
+  const messages3 = [];
+  const onAvailable3 = createMessageCallback(messages3);
+  const onWatcher3Ready = resourceCommand.watchResources([CONSOLE_MESSAGE], {
+    onAvailable: onAvailable3,
+  });
+
+  info("Call unwatchResources for CONSOLE_MESSAGE on watcher 1 & 2");
+
+  info("Wait for all watchers `watchResources` to resolve");
+  await Promise.all([onWatcher1Ready, onWatcher2Ready, onWatcher3Ready]);
+  ok(!hasMessage(messages1, "msg-1"), "Watcher 1 did not receive msg-1");
+  ok(!hasMessage(messages2, "msg-1"), "Watcher 2 did not receive msg-1");
+  ok(hasMessage(messages3, "msg-1"), "Watcher 3 received msg-1");
+
+  info("Log a new message");
+  await logInTab(tab, "msg-2");
+
+  info("Wait until watcher 3 received the new message");
+  await waitUntil(() => hasMessage(messages3, "msg-2"));
+
+  ok(!hasMessage(messages1, "msg-2"), "Watcher 1 did not receive msg-2");
+  ok(!hasMessage(messages2, "msg-2"), "Watcher 2 did not receive msg-2");
+
+  targetCommand.destroy();
+  await client.close();
+});
+
+function logInTab(tab, message) {
+  return ContentTask.spawn(tab.linkedBrowser, message, function (_message) {
+    content.console.log(_message);
+  });
+}
+
+function hasMessage(messageResources, text) {
+  return messageResources.find(
+    resource => resource.message.arguments[0] === text
+  );
+}
+
+// All resource command callbacks share the same pattern here: they add all
+// console message resources to a provided `messages` array.
+function createMessageCallback(messages) {
+  const { CONSOLE_MESSAGE } = ResourceCommand.TYPES;
+  return async resources => {
+    for (const resource of resources) {
+      if (resource.resourceType === CONSOLE_MESSAGE) {
+        messages.push(resource);
+      }
+    }
+  };
+}
diff --git a/devtools/shared/commands/resource/tests/browser_resources_watch_unwatch_multiple.js b/devtools/shared/commands/resource/tests/browser_resources_watch_unwatch_multiple.js
new file mode 100644
index 0000000000..cc45e7bf7f
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/browser_resources_watch_unwatch_multiple.js
@@ -0,0 +1,88 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Check that watching/unwatching multiple times works as expected
+
+add_task(async function () {
+  const TEST_URL = "data:text/html;charset=utf-8,<!DOCTYPE html>foo";
+  const tab = await addTab(TEST_URL);
+
+  const { client, resourceCommand, targetCommand } = await initResourceCommand(
+    tab
+  );
+
+  let resources = [];
+  const onAvailable = _resources => {
+    resources.push(..._resources);
+  };
+
+  info("Watch for error messages resources");
+  await resourceCommand.watchResources([resourceCommand.TYPES.ERROR_MESSAGE], {
+    onAvailable,
+  });
+
+  ok(
+    resourceCommand.isResourceWatched(resourceCommand.TYPES.ERROR_MESSAGE),
+    "The error message resource is currently been watched."
+  );
+
+  is(
+    resources.length,
+    0,
+    "no resources were received after the first watchResources call"
+  );
+
+  info("Trigger an error in the page");
+  await ContentTask.spawn(tab.linkedBrowser, [], function frameScript() {
+    const document = content.document;
+    const scriptEl = document.createElement("script");
+    scriptEl.textContent = `document.unknownFunction()`;
+    document.body.appendChild(scriptEl);
+  });
+
+  await waitFor(() => resources.length === 1);
+  const EXPECTED_ERROR_MESSAGE =
+    "TypeError: document.unknownFunction is not a function";
+  is(
+    resources[0].pageError.errorMessage,
+    EXPECTED_ERROR_MESSAGE,
+    "The resource was received"
+  );
+
+  info("Unwatching resources…");
+  resourceCommand.unwatchResources([resourceCommand.TYPES.ERROR_MESSAGE], {
+    onAvailable,
+  });
+
+  ok(
+    !resourceCommand.isResourceWatched(resourceCommand.TYPES.ERROR_MESSAGE),
+    "The error message resource is no longer been watched."
+  );
+  // clearing resources
+  resources = [];
+
+  info("…and watching again");
+  await resourceCommand.watchResources([resourceCommand.TYPES.ERROR_MESSAGE], {
+    onAvailable,
+  });
+
+  ok(
+    resourceCommand.isResourceWatched(resourceCommand.TYPES.ERROR_MESSAGE),
+    "The error message resource is been watched again."
+  );
+  is(
+    resources.length,
+    1,
+    "we retrieve the expected number of existing resources"
+  );
+  is(
+    resources[0].pageError.errorMessage,
+    EXPECTED_ERROR_MESSAGE,
+    "The resource is the expected one"
+  );
+
+  targetCommand.destroy();
+  await client.close();
+});
diff --git a/devtools/shared/commands/resource/tests/browser_resources_websocket.js b/devtools/shared/commands/resource/tests/browser_resources_websocket.js
new file mode 100644
index 0000000000..601620bc59
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/browser_resources_websocket.js
@@ -0,0 +1,245 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test the ResourceCommand API around WEBSOCKET.
+
+const ResourceCommand = require("resource://devtools/shared/commands/resource/resource-command.js");
+
+const IS_NUMBER = "IS_NUMBER";
+const SHOULD_EXIST = "SHOULD_EXIST";
+
+const targets = {
+  TOP_LEVEL_DOCUMENT: "top-level-document",
+  IN_PROCESS_IFRAME: "in-process-frame",
+  OUT_PROCESS_IFRAME: "out-process-frame",
+};
+
+add_task(async function () {
+  info("Testing the top-level document");
+  await testWebsocketResources(targets.TOP_LEVEL_DOCUMENT);
+  info("Testing the in-process iframe");
+  await testWebsocketResources(targets.IN_PROCESS_IFRAME);
+  info("Testing the out-of-process iframe");
+  await testWebsocketResources(targets.OUT_PROCESS_IFRAME);
+});
+
+async function testWebsocketResources(target) {
+  const tab = await addTab(URL_ROOT_SSL + "websocket_frontend.html");
+  const { client, resourceCommand, targetCommand } = await initResourceCommand(
+    tab
+  );
+
+  const availableResources = [];
+  function onResourceAvailable(resources) {
+    availableResources.push(...resources);
+  }
+
+  await resourceCommand.watchResources([resourceCommand.TYPES.WEBSOCKET], {
+    onAvailable: onResourceAvailable,
+  });
+
+  info("Check available resources at initial");
+  is(
+    availableResources.length,
+    0,
+    "Length of existing resources is correct at initial"
+  );
+
+  info("Check resource of opening websocket");
+  await executeFunctionInContext(tab, target, "openConnection");
+
+  await waitUntil(() => availableResources.length === 1);
+
+  const httpChannelId = availableResources[0].httpChannelId;
+
+  ok(httpChannelId, "httpChannelId is present in the resource");
+
+  assertResource(availableResources[0], {
+    wsMessageType: "webSocketOpened",
+    effectiveURI:
+      "wss://example.com/browser/devtools/shared/commands/resource/tests/websocket_backend",
+    extensions: "permessage-deflate",
+    protocols: "",
+  });
+
+  info("Check resource of sending/receiving the data via websocket");
+  await executeFunctionInContext(tab, target, "sendData", "test");
+
+  await waitUntil(() => availableResources.length === 3);
+
+  assertResource(availableResources[1], {
+    wsMessageType: "frameSent",
+    httpChannelId,
+    data: {
+      type: "sent",
+      payload: "test",
+      timeStamp: SHOULD_EXIST,
+      finBit: SHOULD_EXIST,
+      rsvBit1: SHOULD_EXIST,
+      rsvBit2: SHOULD_EXIST,
+      rsvBit3: SHOULD_EXIST,
+      opCode: SHOULD_EXIST,
+      mask: SHOULD_EXIST,
+      maskBit: SHOULD_EXIST,
+    },
+  });
+  assertResource(availableResources[2], {
+    wsMessageType: "frameReceived",
+    httpChannelId,
+    data: {
+      type: "received",
+      payload: "test",
+      timeStamp: SHOULD_EXIST,
+      finBit: SHOULD_EXIST,
+      rsvBit1: SHOULD_EXIST,
+      rsvBit2: SHOULD_EXIST,
+      rsvBit3: SHOULD_EXIST,
+      opCode: SHOULD_EXIST,
+      mask: SHOULD_EXIST,
+      maskBit: SHOULD_EXIST,
+    },
+  });
+
+  info("Check resource of closing websocket");
+  await executeFunctionInContext(tab, target, "closeConnection");
+
+  await waitUntil(() => availableResources.length === 6);
+  assertResource(availableResources[3], {
+    wsMessageType: "frameSent",
+    httpChannelId,
+    data: {
+      type: "sent",
+      payload: "",
+      timeStamp: SHOULD_EXIST,
+      finBit: SHOULD_EXIST,
+      rsvBit1: SHOULD_EXIST,
+      rsvBit2: SHOULD_EXIST,
+      rsvBit3: SHOULD_EXIST,
+      opCode: SHOULD_EXIST,
+      mask: SHOULD_EXIST,
+      maskBit: SHOULD_EXIST,
+    },
+  });
+  assertResource(availableResources[4], {
+    wsMessageType: "frameReceived",
+    httpChannelId,
+    data: {
+      type: "received",
+      payload: "",
+      timeStamp: SHOULD_EXIST,
+      finBit: SHOULD_EXIST,
+      rsvBit1: SHOULD_EXIST,
+      rsvBit2: SHOULD_EXIST,
+      rsvBit3: SHOULD_EXIST,
+      opCode: SHOULD_EXIST,
+      mask: SHOULD_EXIST,
+      maskBit: SHOULD_EXIST,
+    },
+  });
+  assertResource(availableResources[5], {
+    wsMessageType: "webSocketClosed",
+    httpChannelId,
+    code: IS_NUMBER,
+    reason: "",
+    wasClean: true,
+  });
+
+  info("Check existing resources");
+  const existingResources = [];
+
+  function onExsistingResourceAvailable(resources) {
+    existingResources.push(...resources);
+  }
+
+  await resourceCommand.watchResources([resourceCommand.TYPES.WEBSOCKET], {
+    onAvailable: onExsistingResourceAvailable,
+  });
+
+  is(
+    availableResources.length,
+    existingResources.length,
+    "Length of existing resources is correct"
+  );
+
+  for (let i = 0; i < availableResources.length; i++) {
+    Assert.strictEqual(
+      availableResources[i],
+      existingResources[i],
+      `The ${i}th resource is correct`
+    );
+  }
+
+  await resourceCommand.unwatchResources([resourceCommand.TYPES.WEBSOCKET], {
+    onAvailable: onResourceAvailable,
+  });
+
+  await resourceCommand.unwatchResources([resourceCommand.TYPES.WEBSOCKET], {
+    onAvailable: onExsistingResourceAvailable,
+  });
+
+  targetCommand.destroy();
+  await client.close();
+  BrowserTestUtils.removeTab(tab);
+}
+
+/**
+ * Execute global functions defined in the correct
+ * target (top-level-window or frames) contexts.
+ *
+ * @param {object} tab The current window tab
+ * @param {string} target A string identify if we want to test the top level document or iframes
+ * @param {string} funcName The name of the global function which needs to be called.
+ * @param {*} funcArgs The arguments to pass to the global function
+ */
+async function executeFunctionInContext(tab, target, funcName, ...funcArgs) {
+  let browsingContext = tab.linkedBrowser.browsingContext;
+  // If the target is an iframe get its window global
+  if (target !== targets.TOP_LEVEL_DOCUMENT) {
+    browsingContext = await SpecialPowers.spawn(
+      tab.linkedBrowser,
+      [target],
+      async _target => {
+        const iframe = content.document.getElementById(_target);
+        return iframe.browsingContext;
+      }
+    );
+  }
+
+  return SpecialPowers.spawn(
+    browsingContext,
+    [funcName, funcArgs],
+    async (_funcName, _funcArgs) => {
+      await content.wrappedJSObject[_funcName](..._funcArgs);
+    }
+  );
+}
+
+function assertResource(resource, expected) {
+  is(
+    resource.resourceType,
+    ResourceCommand.TYPES.WEBSOCKET,
+    "Resource type is correct"
+  );
+
+  assertObject(resource, expected);
+}
+
+function assertObject(object, expected) {
+  for (const field in expected) {
+    if (typeof expected[field] === "object") {
+      assertObject(object[field], expected[field]);
+    } else if (expected[field] === SHOULD_EXIST) {
+      Assert.notStrictEqual(
+        object[field],
+        undefined,
+        `The value of ${field} exists`
+      );
+    } else if (expected[field] === IS_NUMBER) {
+      ok(!isNaN(object[field]), `The value of ${field} is number`);
+    } else {
+      is(object[field], expected[field], `The value of ${field} is correct`);
+    }
+  }
+}
diff --git a/devtools/shared/commands/resource/tests/doc_console.html b/devtools/shared/commands/resource/tests/doc_console.html
new file mode 100644
index 0000000000..ee883cf47d
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/doc_console.html
@@ -0,0 +1,18 @@
+<!DOCTYPE HTML>
+<html>
+<head>
+  <meta charset="utf8">
+  <title>Test document for console</title>
+  <!-- Any copyright is dedicated to the Public Domain.
+     - http://creativecommons.org/publicdomain/zero/1.0/ -->
+</head>
+  <body>
+    <p>Test document for console</p>
+
+    <iframe src="data:text/html;charset=utf-8,foo<script>console.log('data url data log')</script>"></iframe>
+    <script>
+    "use strict";
+      console.log("top-level document log");
+    </script>
+  </body>
+</html>
diff --git a/devtools/shared/commands/resource/tests/doc_console_iframe.html b/devtools/shared/commands/resource/tests/doc_console_iframe.html
new file mode 100644
index 0000000000..e088dff4e5
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/doc_console_iframe.html
@@ -0,0 +1,16 @@
+<!DOCTYPE HTML>
+<html>
+<head>
+  <meta charset="utf8">
+  <title>Test fission iframe document</title>
+  <!-- Any copyright is dedicated to the Public Domain.
+     - http://creativecommons.org/publicdomain/zero/1.0/ -->
+</head>
+  <body>
+    <p>remote iframe</p>
+    <script>
+    "use strict";
+      console.log(`${document.location.origin} iframe log`);
+    </script>
+  </body>
+</html>
diff --git a/devtools/shared/commands/resource/tests/early_console_document.html b/devtools/shared/commands/resource/tests/early_console_document.html
new file mode 100644
index 0000000000..e4523dbdeb
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/early_console_document.html
@@ -0,0 +1,14 @@
+<!DOCTYPE HTML>
+<html>
+<head>
+  <meta charset="utf8">
+  <title>Test fission document</title>
+  <!-- Any copyright is dedicated to the Public Domain.
+     - http://creativecommons.org/publicdomain/zero/1.0/ -->
+  <script>
+    "use strict";
+    console.log("early-page-log");
+  </script>
+</head>
+<body></body>
+</html>
diff --git a/devtools/shared/commands/resource/tests/empty.html b/devtools/shared/commands/resource/tests/empty.html
new file mode 100644
index 0000000000..195b296bfe
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/empty.html
@@ -0,0 +1,11 @@
+<!-- Any copyright is dedicated to the Public Domain.
+     http://creativecommons.org/publicdomain/zero/1.0/ -->
+
+<!DOCTYPE HTML>
+<html>
+<head>
+  <meta charset="UTF-8">
+  <title>Empty page (No network requests)</title>
+</head>
+<body></body>
+</html>
diff --git a/devtools/shared/commands/resource/tests/fission_document.html b/devtools/shared/commands/resource/tests/fission_document.html
new file mode 100644
index 0000000000..222f92d999
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/fission_document.html
@@ -0,0 +1,23 @@
+<!DOCTYPE HTML>
+<html>
+<head>
+  <meta charset="utf8">
+  <title>Test fission document</title>
+  <!-- Any copyright is dedicated to the Public Domain.
+     - http://creativecommons.org/publicdomain/zero/1.0/ -->
+</head>
+<body>
+<p>Test fission iframe</p>
+
+<script>
+  "use strict";
+  const iframe = document.createElement("iframe");
+  let iframeUrl = `https://example.org/browser/devtools/shared/commands/resource/tests/fission_iframe.html`;
+  if (document.location.search) {
+    iframeUrl += `?${new URLSearchParams(document.location.search)}`;
+  }
+  iframe.src = iframeUrl;
+  document.body.append(iframe);
+</script>
+</body>
+</html>
diff --git a/devtools/shared/commands/resource/tests/fission_document_workers.html b/devtools/shared/commands/resource/tests/fission_document_workers.html
new file mode 100644
index 0000000000..bbbe3e8bf8
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/fission_document_workers.html
@@ -0,0 +1,47 @@
+<!DOCTYPE HTML>
+<html>
+<head>
+  <meta charset="utf8">
+  <title>Test fission document</title>
+  <!-- Any copyright is dedicated to the Public Domain.
+     - http://creativecommons.org/publicdomain/zero/1.0/ -->
+  <script>
+  "use strict";
+
+  const params = new URLSearchParams(document.location.search);
+
+  // eslint-disable-next-line no-unused-vars
+  const worker = new Worker("https://example.com/browser/devtools/shared/commands/resource/tests/test_worker.js#simple-worker");
+
+  // eslint-disable-next-line no-unused-vars
+  const sharedWorker = new SharedWorker("https://example.com/browser/devtools/shared/commands/resource/tests/test_worker.js#shared-worker");
+
+  if (!params.has("noServiceWorker")) {
+    // Expose a reference to the registration so that tests can unregister it.
+    window.registrationPromise = navigator.serviceWorker.register("https://example.com/browser/devtools/shared/commands/resource/tests/test_service_worker.js#service-worker");
+  }
+
+  /* exported logMessageInWorker */
+  function logMessageInWorker(message) {
+    worker.postMessage({
+      type: "log-in-worker",
+      message,
+    });
+  }
+  </script>
+</head>
+<body>
+<p>Test fission iframe</p>
+
+<script>
+  "use strict";
+  const iframe = document.createElement("iframe");
+  let iframeUrl = `https://example.org/browser/devtools/shared/commands/resource/tests/fission_iframe_workers.html`;
+  if (document.location.search) {
+    iframeUrl += `?${new URLSearchParams(document.location.search)}`;
+  }
+  iframe.src = iframeUrl;
+  document.body.append(iframe);
+</script>
+</body>
+</html>
diff --git a/devtools/shared/commands/resource/tests/fission_iframe.html b/devtools/shared/commands/resource/tests/fission_iframe.html
new file mode 100644
index 0000000000..f674321102
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/fission_iframe.html
@@ -0,0 +1,12 @@
+<!DOCTYPE HTML>
+<html>
+<head>
+  <meta charset="utf8">
+  <title>Test fission iframe document</title>
+  <!-- Any copyright is dedicated to the Public Domain.
+     - http://creativecommons.org/publicdomain/zero/1.0/ -->
+</head>
+<body>
+<p>remote iframe</p>
+</body>
+</html>
diff --git a/devtools/shared/commands/resource/tests/fission_iframe_workers.html b/devtools/shared/commands/resource/tests/fission_iframe_workers.html
new file mode 100644
index 0000000000..deae49f833
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/fission_iframe_workers.html
@@ -0,0 +1,29 @@
+<!DOCTYPE HTML>
+<html>
+<head>
+  <meta charset="utf8">
+  <title>Test fission iframe document</title>
+  <!-- Any copyright is dedicated to the Public Domain.
+     - http://creativecommons.org/publicdomain/zero/1.0/ -->
+  <script>
+    "use strict";
+    const params = new URLSearchParams(document.location.search);
+    const hashSuffix = params.get("hashSuffix") || "in-iframe";
+    // eslint-disable-next-line no-unused-vars
+    const worker = new Worker("test_worker.js#simple-worker-" + hashSuffix);
+    // eslint-disable-next-line no-unused-vars
+    const sharedWorker = new SharedWorker("test_worker.js#shared-worker-" + hashSuffix);
+
+    /* exported logMessageInWorker */
+    function logMessageInWorker(message) {
+      worker.postMessage({
+        type: "log-in-worker",
+        message,
+      });
+    }
+  </script>
+</head>
+<body>
+<p>remote iframe</p>
+</body>
+</html>
diff --git a/devtools/shared/commands/resource/tests/head.js b/devtools/shared/commands/resource/tests/head.js
new file mode 100644
index 0000000000..5cee383070
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/head.js
@@ -0,0 +1,151 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+/* eslint no-unused-vars: [2, {"vars": "local"}] */
+
+Services.scriptloader.loadSubScript(
+  "chrome://mochitests/content/browser/devtools/client/shared/test/shared-head.js",
+  this
+);
+
+const {
+  DevToolsClient,
+} = require("resource://devtools/client/devtools-client.js");
+const {
+  DevToolsServer,
+} = require("resource://devtools/server/devtools-server.js");
+
+async function _initResourceCommandFromCommands(
+  commands,
+  { listenForWorkers = false } = {}
+) {
+  const targetCommand = commands.targetCommand;
+  if (listenForWorkers) {
+    targetCommand.listenForWorkers = true;
+  }
+  await targetCommand.startListening();
+
+  //Bug 1709065: Stop exporting resourceCommand and use commands.resourceCommand
+  //And rename all these methods
+  return {
+    client: commands.client,
+    commands,
+    resourceCommand: commands.resourceCommand,
+    targetCommand,
+  };
+}
+
+/**
+ * Instantiate a ResourceCommand for the given tab.
+ *
+ * @param {Tab} tab
+ *        The browser frontend's tab to connect to.
+ * @param {Object} options
+ * @param {Boolean} options.listenForWorkers
+ * @return {Object} object
+ * @return {ResourceCommand} object.resourceCommand
+ *         The underlying resource command interface.
+ * @return {Object} object.commands
+ *         The commands object defined by modules from devtools/shared/commands.
+ * @return {DevToolsClient} object.client
+ *         The underlying client instance.
+ * @return {TargetCommand} object.targetCommand
+ *         The underlying target list instance.
+ */
+async function initResourceCommand(tab, options) {
+  const commands = await CommandsFactory.forTab(tab);
+  return _initResourceCommandFromCommands(commands, options);
+}
+
+/**
+ * Instantiate a multi-process ResourceCommand, watching all type of targets.
+ *
+ * @return {Object} object
+ * @return {ResourceCommand} object.resourceCommand
+ *         The underlying resource command interface.
+ * @return {Object} object.commands
+ *         The commands object defined by modules from devtools/shared/commands.
+ * @return {DevToolsClient} object.client
+ *         The underlying client instance.
+ * @return {DevToolsClient} object.targetCommand
+ *         The underlying target list instance.
+ */
+async function initMultiProcessResourceCommand() {
+  const commands = await CommandsFactory.forMainProcess();
+  return _initResourceCommandFromCommands(commands);
+}
+
+// Copied from devtools/shared/webconsole/test/chrome/common.js
+function checkObject(object, expected) {
+  if (object && object.getGrip) {
+    object = object.getGrip();
+  }
+
+  for (const name of Object.keys(expected)) {
+    const expectedValue = expected[name];
+    const value = object[name];
+    checkValue(name, value, expectedValue);
+  }
+}
+
+function checkValue(name, value, expected) {
+  if (expected === null) {
+    is(value, null, `'${name}' is null`);
+  } else if (expected === undefined) {
+    is(value, expected, `'${name}' is undefined`);
+  } else if (
+    typeof expected == "string" ||
+    typeof expected == "number" ||
+    typeof expected == "boolean"
+  ) {
+    is(value, expected, "property '" + name + "'");
+  } else if (expected instanceof RegExp) {
+    ok(expected.test(value), name + ": " + expected + " matched " + value);
+  } else if (Array.isArray(expected)) {
+    info("checking array for property '" + name + "'");
+    ok(Array.isArray(value), `property '${name}' is an array`);
+
+    is(value.length, expected.length, "Array has expected length");
+    if (value.length !== expected.length) {
+      is(JSON.stringify(value, null, 2), JSON.stringify(expected, null, 2));
+    } else {
+      checkObject(value, expected);
+    }
+  } else if (typeof expected == "object") {
+    info("checking object for property '" + name + "'");
+    checkObject(value, expected);
+  }
+}
+
+async function triggerNetworkRequests(browser, commands) {
+  for (let i = 0; i < commands.length; i++) {
+    await SpecialPowers.spawn(browser, [commands[i]], async function (code) {
+      const script = content.document.createElement("script");
+      script.append(
+        content.document.createTextNode(
+          `async function triggerRequest() {${code}}`
+        )
+      );
+      content.document.body.append(script);
+      await content.wrappedJSObject.triggerRequest();
+      script.remove();
+    });
+  }
+}
+
+/**
+ * Get the stylesheet text for a given stylesheet resource.
+ *
+ * @param {Object} styleSheetResource
+ * @returns Promise<String>
+ */
+async function getStyleSheetResourceText(styleSheetResource) {
+  const styleSheetsFront = await styleSheetResource.targetFront.getFront(
+    "stylesheets"
+  );
+  const res = await styleSheetsFront.getText(styleSheetResource.resourceId);
+  return res.string();
+}
diff --git a/devtools/shared/commands/resource/tests/network_document.html b/devtools/shared/commands/resource/tests/network_document.html
new file mode 100644
index 0000000000..5c4744cb0c
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/network_document.html
@@ -0,0 +1,13 @@
+<!DOCTYPE html>
+<html lang="en">
+  <head>
+    <meta charset="utf-8">
+    <meta http-equiv="Cache-Control" content="no-cache, no-store, must-revalidate" />
+    <meta http-equiv="Pragma" content="no-cache" />
+    <meta http-equiv="Expires" content="0" />
+    <title>Test for network events</title>
+  </head>
+  <body>
+    <p>Test for network events</p>
+  </body>
+</html>
diff --git a/devtools/shared/commands/resource/tests/network_document_navigation.html b/devtools/shared/commands/resource/tests/network_document_navigation.html
new file mode 100644
index 0000000000..c4ec651c05
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/network_document_navigation.html
@@ -0,0 +1,14 @@
+<!DOCTYPE html>
+<html lang="en">
+  <head>
+    <meta charset="utf-8">
+    <meta http-equiv="Cache-Control" content="no-cache, no-store, must-revalidate" />
+    <meta http-equiv="Pragma" content="no-cache" />
+    <meta http-equiv="Expires" content="0" />
+    <title>Test for network events</title>
+  </head>
+  <body>
+    <p>Test for network events</p>
+    <script src="network_navigation.js" />
+  </body>
+</html>
diff --git a/devtools/shared/commands/resource/tests/network_navigation.js b/devtools/shared/commands/resource/tests/network_navigation.js
new file mode 100644
index 0000000000..6004b69d3c
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/network_navigation.js
@@ -0,0 +1 @@
+// empty script loaded by network_document_navigation.html
diff --git a/devtools/shared/commands/resource/tests/service-worker-sources.js b/devtools/shared/commands/resource/tests/service-worker-sources.js
new file mode 100644
index 0000000000..614644ee5d
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/service-worker-sources.js
@@ -0,0 +1,2 @@
+/* eslint-disable */
+function serviceWorkerSource() {}
diff --git a/devtools/shared/commands/resource/tests/sources.html b/devtools/shared/commands/resource/tests/sources.html
new file mode 100644
index 0000000000..9e1ad67d85
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/sources.html
@@ -0,0 +1,53 @@
+<!-- Any copyright is dedicated to the Public Domain.
+     http://creativecommons.org/publicdomain/zero/1.0/ -->
+<!doctype HTML>
+<html>
+  <head>
+    <meta charset="utf-8"/>
+  </head>
+  <body>
+    <!-- introductionType=eventHandler -->
+    <div onclick="console.log('link')">link</div>
+
+    <!-- introductionType=inlineScript mapped to scriptElement -->
+    <script type="text/javascript">
+      "use strict";
+      /* eslint-disable */
+      function inlineSource() {}
+
+      // introductionType=eval
+      // Assign it to a global in order to avoid it being GCed
+      eval("this.global = function evalFunction() {}");
+
+      // introductionType=Function
+      // Also assign to a global to avoid being GCed
+      this.global2 = new Function("return 42;");
+
+      // introductionType=injectedScript mapped to scriptElement
+      const script = document.createElement("script");
+      script.textContent = "console.log('inline-script')";
+      document.documentElement.appendChild(script);
+
+      // introductionType=Worker, but ends up being null on SourceActor's form
+      // Assign the worker to a global variable in order to avoid
+      // having it be GCed.
+      this.worker = new Worker("worker-sources.js");
+
+      window.registrationPromise = navigator.serviceWorker.register("service-worker-sources.js");
+
+      // introductionType=domTimer
+      setTimeout(`console.log("timeout")`, 0);
+
+      // introductionType=eventHandler
+      window.addEventListener("DOMContentLoaded", () => {
+        document.querySelector("div[onclick]").click();
+      });
+    </script>
+    <!-- introductionType=srcScript mapped to scriptElement -->
+    <script src="sources.js"></script>
+    <!-- introductionType=javascriptURL -->
+    <iframe src="javascript:666"></iframe>
+    <!-- srcdoc attribute on iframes -->
+    <iframe srcdoc="<script>console.log('srcdoc')</script> <script>console.log('srcdoc 2')</script>"></iframe>
+  </body>
+</html>
diff --git a/devtools/shared/commands/resource/tests/sources.js b/devtools/shared/commands/resource/tests/sources.js
new file mode 100644
index 0000000000..7ae6c6272b
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/sources.js
@@ -0,0 +1,2 @@
+/* eslint-disable */
+function scriptSource() {}
diff --git a/devtools/shared/commands/resource/tests/sse_backend.sjs b/devtools/shared/commands/resource/tests/sse_backend.sjs
new file mode 100644
index 0000000000..777520577a
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/sse_backend.sjs
@@ -0,0 +1,8 @@
+"use strict";
+
+function handleRequest(request, response) {
+  response.processAsync();
+  response.setHeader("Content-Type", "text/event-stream");
+  response.write("data: Why so serious?\n\n");
+  response.finish();
+}
diff --git a/devtools/shared/commands/resource/tests/sse_frontend.html b/devtools/shared/commands/resource/tests/sse_frontend.html
new file mode 100644
index 0000000000..3bdddbc5bc
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/sse_frontend.html
@@ -0,0 +1,31 @@
+<!-- Any copyright is dedicated to the Public Domain.
+     http://creativecommons.org/publicdomain/zero/1.0/ -->
+<!doctype HTML>
+<html>
+  <head>
+    <meta charset="utf-8"/>
+    <meta http-equiv="Cache-Control" content="no-cache, no-store, must-revalidate" />
+    <meta http-equiv="Pragma" content="no-cache" />
+    <meta http-equiv="Expires" content="0" />
+    <title>SSE Inspection Test Page</title>
+  </head>
+  <body>
+    <h1>SSE Inspection Test Page</h1>
+    <script type="text/javascript">
+      "use strict";
+
+      /* exported openConnection */
+      function openConnection() {
+        return new Promise(resolve => {
+          const es = new EventSource("sse_backend.sjs");
+          es.onmessage = function (e) {
+            es.close();
+            resolve();
+          };
+        });
+      }
+    </script>
+    <iframe id="in-process-frame"  src="https://example.com/browser/devtools/shared/commands/resource/tests/sse_frontend_iframe.html"> </iframe>
+    <iframe id="out-process-frame" src="https://example.org/browser/devtools/shared/commands/resource/tests/sse_frontend_iframe.html"></iframe>
+  </body>
+</html>
diff --git a/devtools/shared/commands/resource/tests/sse_frontend_iframe.html b/devtools/shared/commands/resource/tests/sse_frontend_iframe.html
new file mode 100644
index 0000000000..477dca013d
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/sse_frontend_iframe.html
@@ -0,0 +1,29 @@
+<!-- Any copyright is dedicated to the Public Domain.
+     http://creativecommons.org/publicdomain/zero/1.0/ -->
+<!doctype HTML>
+<html>
+  <head>
+      <meta charset="utf-8"/>
+      <meta http-equiv="Cache-Control" content="no-cache, no-store, must-revalidate" />
+      <meta http-equiv="Pragma" content="no-cache" />
+      <meta http-equiv="Expires" content="0" />
+      <title>SSE Inspection Test Page in iframe</title>
+  </head>
+  <body>
+    <h1>SSE Inspection Test Page in Iframe</h1>
+    <script type="text/javascript">
+      "use strict";
+
+      /* exported openConnection */
+      function openConnection() {
+        return new Promise(resolve => {
+          const es = new EventSource("sse_backend.sjs");
+          es.onmessage = function (e) {
+            es.close();
+            resolve();
+          };
+        });
+      }
+    </script>
+  </body>
+</html>
diff --git a/devtools/shared/commands/resource/tests/style_document.css b/devtools/shared/commands/resource/tests/style_document.css
new file mode 100644
index 0000000000..aa54533924
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/style_document.css
@@ -0,0 +1 @@
+body { margin: 1px; }
diff --git a/devtools/shared/commands/resource/tests/style_document.html b/devtools/shared/commands/resource/tests/style_document.html
new file mode 100644
index 0000000000..deaf6c4248
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/style_document.html
@@ -0,0 +1,22 @@
+<!DOCTYPE HTML>
+<html>
+  <head>
+    <meta charset="utf8">
+    <title>Test style document</title>
+    <!-- Any copyright is dedicated to the Public Domain.
+       - http://creativecommons.org/publicdomain/zero/1.0/ -->
+    <style>
+    body { color: lime; }
+    </style>
+    <link href="style_document.css" rel="stylesheet">
+    <script>
+      "use strict";
+      const s = new CSSStyleSheet();
+      s.replaceSync("body { background-color: blue }");
+      document.adoptedStyleSheets.push(s);
+    </script>
+  </head>
+  <body>
+    <iframe src="https://example.org/browser/devtools/shared/commands/resource/tests/style_iframe.html"></iframe>
+  </body>
+</html>
diff --git a/devtools/shared/commands/resource/tests/style_iframe.css b/devtools/shared/commands/resource/tests/style_iframe.css
new file mode 100644
index 0000000000..30e7ae802b
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/style_iframe.css
@@ -0,0 +1 @@
+body { padding: 1px; }
diff --git a/devtools/shared/commands/resource/tests/style_iframe.html b/devtools/shared/commands/resource/tests/style_iframe.html
new file mode 100644
index 0000000000..11ad9f785b
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/style_iframe.html
@@ -0,0 +1,15 @@
+<!DOCTYPE HTML>
+<html>
+  <head>
+    <meta charset="utf8">
+    <title>Test style iframe document</title>
+    <!-- Any copyright is dedicated to the Public Domain.
+       - http://creativecommons.org/publicdomain/zero/1.0/ -->
+    <style>
+    body { background-color: pink; }
+    </style>
+    <link href="style_iframe.css" rel="stylesheet" type="text/css"/>
+  </head>
+  <body>
+  </body>
+</html>
diff --git a/devtools/shared/commands/resource/tests/stylesheets-nested-iframes.html b/devtools/shared/commands/resource/tests/stylesheets-nested-iframes.html
new file mode 100644
index 0000000000..eb6c371867
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/stylesheets-nested-iframes.html
@@ -0,0 +1,27 @@
+<!DOCTYPE HTML>
+<html>
+<head>
+  <meta charset="utf-8">
+  <title>StyleSheetsActor iframe test</title>
+  <style>
+    p {
+      padding: 1em;
+    }
+  </style>
+</head>
+<body>
+  <p>A test page with nested iframes</p>
+  <iframe></iframe>
+  <script type="application/javascript">
+    "use strict";
+
+    const iframe = document.querySelector("iframe");
+    let i = parseInt(location.href.split("?")[1], 10) || 1;
+
+    // The frame can't have the same src URL as any of its ancestors.
+    // This will not infinitely recurse because a frame won't get a content
+    // document once it's nested deeply enough.
+    iframe.src = location.href.split("?")[0] + "?" + (++i);
+  </script>
+</body>
+</html>
diff --git a/devtools/shared/commands/resource/tests/test_image.png b/devtools/shared/commands/resource/tests/test_image.png
new file mode 100644
index 0000000000..769c636340
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/test_image.png
diff --git a/devtools/shared/commands/resource/tests/test_service_worker.js b/devtools/shared/commands/resource/tests/test_service_worker.js
new file mode 100644
index 0000000000..aabc3fda0f
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/test_service_worker.js
@@ -0,0 +1,11 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// We don't need any computation in the worker,
+// but at least register a fetch listener so that
+// we force instantiating the SW when loading the page.
+self.onfetch = function (event) {
+  // do nothing.
+};
diff --git a/devtools/shared/commands/resource/tests/test_worker.js b/devtools/shared/commands/resource/tests/test_worker.js
new file mode 100644
index 0000000000..60ccc6d52b
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/test_worker.js
@@ -0,0 +1,15 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+console.log("[WORKER] started", globalThis.location.toString(), globalThis);
+
+globalThis.onmessage = function (e) {
+  const { type, message } = e.data;
+
+  if (type === "log-in-worker") {
+    // Printing `e` so we can check that we have an object and not a stringified version
+    console.log("[WORKER]", message, e);
+  }
+};
diff --git a/devtools/shared/commands/resource/tests/websocket_backend_wsh.py b/devtools/shared/commands/resource/tests/websocket_backend_wsh.py
new file mode 100644
index 0000000000..170f15fe6c
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/websocket_backend_wsh.py
@@ -0,0 +1,20 @@
+from mod_pywebsocket import msgutil
+
+
+def web_socket_do_extra_handshake(request):
+    pass
+
+
+def web_socket_transfer_data(request):
+    while not request.client_terminated:
+        resp = msgutil.receive_message(request)
+        msgutil.send_message(request, resp)
+
+
+def web_socket_passive_closing_handshake(request):
+    # If we use `pass` here, the `payload` of `frameReceived` which will be happened
+    # of communication of closing will be `\u0003è`. In order to make the `payload`
+    # to be empty string, return code and reason explicitly.
+    code = None
+    reason = None
+    return code, reason
diff --git a/devtools/shared/commands/resource/tests/websocket_frontend.html b/devtools/shared/commands/resource/tests/websocket_frontend.html
new file mode 100644
index 0000000000..7efe11f9eb
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/websocket_frontend.html
@@ -0,0 +1,45 @@
+<!-- Any copyright is dedicated to the Public Domain.
+     http://creativecommons.org/publicdomain/zero/1.0/ -->
+<!doctype HTML>
+<html>
+  <head>
+    <meta charset="utf-8" />
+    <title>Websocket Inspection Test Page</title>
+  </head>
+  <body>
+    <h1>Websocket Inspection Test Page</h1>
+    <script type="text/javascript">
+      /* exported openConnection, closeConnection, sendData */
+      "use strict";
+
+      let webSocket;
+      function openConnection() {
+        return new Promise(resolve => {
+          webSocket = new WebSocket(
+            "wss://example.com/browser/devtools/shared/commands/resource/tests/websocket_backend"
+          );
+          webSocket.onopen = () => {
+            resolve();
+          };
+        });
+      }
+
+      function closeConnection() {
+        return new Promise(resolve => {
+          webSocket.onclose = () => {
+            resolve();
+          };
+          webSocket.close();
+        })
+      }
+
+      function sendData(payload) {
+        webSocket.send(payload);
+      }
+    </script>
+    <iframe id="in-process-frame"
+      src="https://example.com/browser/devtools/shared/commands/resource/tests/websocket_frontend_iframe.html"></iframe>
+    <iframe id="out-process-frame"
+      src="https://example.org/browser/devtools/shared/commands/resource/tests/websocket_frontend_iframe.html"></iframe>
+  </body>
+</html>
diff --git a/devtools/shared/commands/resource/tests/websocket_frontend_iframe.html b/devtools/shared/commands/resource/tests/websocket_frontend_iframe.html
new file mode 100644
index 0000000000..e18576f911
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/websocket_frontend_iframe.html
@@ -0,0 +1,41 @@
+<!-- Any copyright is dedicated to the Public Domain.
+     http://creativecommons.org/publicdomain/zero/1.0/ -->
+<!doctype HTML>
+<html>
+  <head>
+    <meta charset="utf-8"/>
+    <title>Websocket Inspection Test Page</title>
+  </head>
+  <body>
+    <h1>Websocket Inspection Test Page</h1>
+    <script type="text/javascript">
+    /* exported openConnection, closeConnection, sendData */
+    "use strict";
+
+    let webSocket;
+    function openConnection() {
+      return new Promise(resolve => {
+        webSocket = new WebSocket(
+          "wss://example.com/browser/devtools/shared/commands/resource/tests/websocket_backend"
+        );
+        webSocket.onopen = () => {
+          resolve();
+        };
+      });
+    }
+
+    function closeConnection() {
+      return new Promise(resolve => {
+        webSocket.onclose = () => {
+          resolve();
+        };
+        webSocket.close();
+      })
+    }
+
+    function sendData(payload) {
+      webSocket.send(payload);
+    }
+    </script>
+  </body>
+</html>
diff --git a/devtools/shared/commands/resource/tests/worker-sources.js b/devtools/shared/commands/resource/tests/worker-sources.js
new file mode 100644
index 0000000000..dcf2ed8031
--- /dev/null
+++ b/devtools/shared/commands/resource/tests/worker-sources.js
@@ -0,0 +1,2 @@
+/* eslint-disable */
+function workerSource() {}
diff --git a/devtools/shared/commands/resource/transformers/console-messages.js b/devtools/shared/commands/resource/transformers/console-messages.js
new file mode 100644
index 0000000000..9c8ca51f04
--- /dev/null
+++ b/devtools/shared/commands/resource/transformers/console-messages.js
@@ -0,0 +1,23 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+// eslint-disable-next-line mozilla/reject-some-requires
+loader.lazyRequireGetter(
+  this,
+  "getAdHocFrontOrPrimitiveGrip",
+  "resource://devtools/client/fronts/object.js",
+  true
+);
+
+module.exports = function ({ resource, targetFront }) {
+  if (Array.isArray(resource.message.arguments)) {
+    // We might need to create fronts for each of the message arguments.
+    resource.message.arguments = resource.message.arguments.map(arg =>
+      getAdHocFrontOrPrimitiveGrip(arg, targetFront)
+    );
+  }
+  return resource;
+};
diff --git a/devtools/shared/commands/resource/transformers/error-messages.js b/devtools/shared/commands/resource/transformers/error-messages.js
new file mode 100644
index 0000000000..2b71f5b7ca
--- /dev/null
+++ b/devtools/shared/commands/resource/transformers/error-messages.js
@@ -0,0 +1,31 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+// eslint-disable-next-line mozilla/reject-some-requires
+loader.lazyRequireGetter(
+  this,
+  "getAdHocFrontOrPrimitiveGrip",
+  "resource://devtools/client/fronts/object.js",
+  true
+);
+
+module.exports = function ({ resource, targetFront }) {
+  if (resource?.pageError?.errorMessage) {
+    resource.pageError.errorMessage = getAdHocFrontOrPrimitiveGrip(
+      resource.pageError.errorMessage,
+      targetFront
+    );
+  }
+
+  if (resource?.pageError?.exception) {
+    resource.pageError.exception = getAdHocFrontOrPrimitiveGrip(
+      resource.pageError.exception,
+      targetFront
+    );
+  }
+
+  return resource;
+};
diff --git a/devtools/shared/commands/resource/transformers/moz.build b/devtools/shared/commands/resource/transformers/moz.build
new file mode 100644
index 0000000000..5b0b94853a
--- /dev/null
+++ b/devtools/shared/commands/resource/transformers/moz.build
@@ -0,0 +1,16 @@
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+DevToolsModules(
+    "console-messages.js",
+    "error-messages.js",
+    "network-events.js",
+    "storage-cache.js",
+    "storage-cookie.js",
+    "storage-extension.js",
+    "storage-indexed-db.js",
+    "storage-local-storage.js",
+    "storage-session-storage.js",
+    "thread-states.js",
+)
diff --git a/devtools/shared/commands/resource/transformers/network-events.js b/devtools/shared/commands/resource/transformers/network-events.js
new file mode 100644
index 0000000000..d7f757d706
--- /dev/null
+++ b/devtools/shared/commands/resource/transformers/network-events.js
@@ -0,0 +1,16 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+const {
+  getUrlDetails,
+  // eslint-disable-next-line mozilla/reject-some-requires
+} = require("resource://devtools/client/netmonitor/src/utils/request-utils.js");
+
+module.exports = function ({ resource }) {
+  resource.urlDetails = getUrlDetails(resource.url);
+  resource.startedMs = Date.parse(resource.startedDateTime);
+  return resource;
+};
diff --git a/devtools/shared/commands/resource/transformers/storage-cache.js b/devtools/shared/commands/resource/transformers/storage-cache.js
new file mode 100644
index 0000000000..245d892041
--- /dev/null
+++ b/devtools/shared/commands/resource/transformers/storage-cache.js
@@ -0,0 +1,22 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+const {
+  TYPES: { CACHE_STORAGE },
+} = require("resource://devtools/shared/commands/resource/resource-command.js");
+
+const { Front, types } = require("resource://devtools/shared/protocol.js");
+
+module.exports = function ({ resource, watcherFront, targetFront }) {
+  if (!(resource instanceof Front) && watcherFront) {
+    // instantiate front for local storage
+    resource = types.getType("Cache").read(resource, targetFront);
+    resource.resourceType = CACHE_STORAGE;
+    resource.resourceKey = "Cache";
+  }
+
+  return resource;
+};
diff --git a/devtools/shared/commands/resource/transformers/storage-cookie.js b/devtools/shared/commands/resource/transformers/storage-cookie.js
new file mode 100644
index 0000000000..23e221672b
--- /dev/null
+++ b/devtools/shared/commands/resource/transformers/storage-cookie.js
@@ -0,0 +1,26 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+const {
+  TYPES: { COOKIE },
+} = require("resource://devtools/shared/commands/resource/resource-command.js");
+
+const { Front, types } = require("resource://devtools/shared/protocol.js");
+
+module.exports = function ({ resource, watcherFront, targetFront }) {
+  if (!(resource instanceof Front) && watcherFront) {
+    const { innerWindowId } = resource;
+
+    // it's safe to instantiate the front now, so we do it.
+    resource = types.getType("cookies").read(resource, targetFront);
+    resource.resourceType = COOKIE;
+    resource.resourceId = `${COOKIE}-${targetFront.browsingContextID}`;
+    resource.resourceKey = "cookies";
+    resource.innerWindowId = innerWindowId;
+  }
+
+  return resource;
+};
diff --git a/devtools/shared/commands/resource/transformers/storage-extension.js b/devtools/shared/commands/resource/transformers/storage-extension.js
new file mode 100644
index 0000000000..3e40bdd6d0
--- /dev/null
+++ b/devtools/shared/commands/resource/transformers/storage-extension.js
@@ -0,0 +1,26 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+const {
+  TYPES: { EXTENSION_STORAGE },
+} = require("resource://devtools/shared/commands/resource/resource-command.js");
+
+const { Front, types } = require("resource://devtools/shared/protocol.js");
+
+module.exports = function ({ resource, watcherFront, targetFront }) {
+  if (!(resource instanceof Front) && watcherFront) {
+    const { innerWindowId } = resource;
+
+    // it's safe to instantiate the front now, so we do it.
+    resource = types.getType("extensionStorage").read(resource, targetFront);
+    resource.resourceType = EXTENSION_STORAGE;
+    resource.resourceId = `${EXTENSION_STORAGE}-${targetFront.browsingContextID}`;
+    resource.resourceKey = "extensionStorage";
+    resource.innerWindowId = innerWindowId;
+  }
+
+  return resource;
+};
diff --git a/devtools/shared/commands/resource/transformers/storage-indexed-db.js b/devtools/shared/commands/resource/transformers/storage-indexed-db.js
new file mode 100644
index 0000000000..8021719070
--- /dev/null
+++ b/devtools/shared/commands/resource/transformers/storage-indexed-db.js
@@ -0,0 +1,26 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+const {
+  TYPES: { INDEXED_DB },
+} = require("resource://devtools/shared/commands/resource/resource-command.js");
+
+const { Front, types } = require("resource://devtools/shared/protocol.js");
+
+module.exports = function ({ resource, watcherFront, targetFront }) {
+  if (!(resource instanceof Front) && watcherFront) {
+    const { innerWindowId } = resource;
+
+    // it's safe to instantiate the front now, so we do it.
+    resource = types.getType("indexedDB").read(resource, targetFront);
+    resource.resourceType = INDEXED_DB;
+    resource.resourceId = `${INDEXED_DB}-${targetFront.browsingContextID}`;
+    resource.resourceKey = "indexedDB";
+    resource.innerWindowId = innerWindowId;
+  }
+
+  return resource;
+};
diff --git a/devtools/shared/commands/resource/transformers/storage-local-storage.js b/devtools/shared/commands/resource/transformers/storage-local-storage.js
new file mode 100644
index 0000000000..13488723f3
--- /dev/null
+++ b/devtools/shared/commands/resource/transformers/storage-local-storage.js
@@ -0,0 +1,22 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+const {
+  TYPES: { LOCAL_STORAGE },
+} = require("resource://devtools/shared/commands/resource/resource-command.js");
+
+const { Front, types } = require("resource://devtools/shared/protocol.js");
+
+module.exports = function ({ resource, watcherFront, targetFront }) {
+  if (!(resource instanceof Front) && watcherFront) {
+    // instantiate front for local storage
+    resource = types.getType("localStorage").read(resource, targetFront);
+    resource.resourceType = LOCAL_STORAGE;
+    resource.resourceKey = "localStorage";
+  }
+
+  return resource;
+};
diff --git a/devtools/shared/commands/resource/transformers/storage-session-storage.js b/devtools/shared/commands/resource/transformers/storage-session-storage.js
new file mode 100644
index 0000000000..ab9f1361c8
--- /dev/null
+++ b/devtools/shared/commands/resource/transformers/storage-session-storage.js
@@ -0,0 +1,22 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+const {
+  TYPES: { SESSION_STORAGE },
+} = require("resource://devtools/shared/commands/resource/resource-command.js");
+
+const { Front, types } = require("resource://devtools/shared/protocol.js");
+
+module.exports = function ({ resource, watcherFront, targetFront }) {
+  if (!(resource instanceof Front) && watcherFront) {
+    // instantiate front for session storage
+    resource = types.getType("sessionStorage").read(resource, targetFront);
+    resource.resourceType = SESSION_STORAGE;
+    resource.resourceKey = "sessionStorage";
+  }
+
+  return resource;
+};
diff --git a/devtools/shared/commands/resource/transformers/thread-states.js b/devtools/shared/commands/resource/transformers/thread-states.js
new file mode 100644
index 0000000000..1564585b36
--- /dev/null
+++ b/devtools/shared/commands/resource/transformers/thread-states.js
@@ -0,0 +1,32 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+const { Front, types } = require("resource://devtools/shared/protocol.js");
+
+module.exports = function ({ resource, watcherFront, targetFront }) {
+  // only "paused" have a frame attribute, and legacy listeners are already passing a FrameFront
+  if (resource.frame && !(resource.frame instanceof Front)) {
+    // Use ThreadFront as parent as debugger's commands.js expects FrameFront to be children
+    // of the ThreadFront.
+    resource.frame = types
+      .getType("frame")
+      .read(resource.frame, targetFront.threadFront);
+  }
+
+  // If we are using server side request (i.e. watcherFront is defined)
+  // Fake paused and resumed events as the thread front depends on them.
+  // We can't emit "EventEmitter" events, as ThreadFront uses `Front.before`
+  // to listen for paused and resumed. ("before" is part of protocol.js Front and not part of EventEmitter)
+  if (watcherFront) {
+    if (resource.state == "paused") {
+      targetFront.threadFront._beforePaused(resource);
+    } else if (resource.state == "resumed") {
+      targetFront.threadFront._beforeResumed(resource);
+    }
+  }
+
+  return resource;
+};