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diff --git a/devtools/server/actors/watcher/WatcherRegistry.jsm b/devtools/server/actors/watcher/WatcherRegistry.jsm
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+/* 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";
+
+/**
+ * Helper module around `sharedData` object that helps storing the state
+ * of all observed Targets and Resources, that, for all DevTools connections.
+ * Here is a few words about the C++ implementation of sharedData:
+ * https://searchfox.org/mozilla-central/rev/bc3600def806859c31b2c7ac06e3d69271052a89/dom/ipc/SharedMap.h#30-55
+ *
+ * We may have more than one DevToolsServer and one server may have more than one
+ * client. This module will be the single source of truth in the parent process,
+ * in order to know which targets/resources are currently observed. It will also
+ * be used to declare when something starts/stops being observed.
+ *
+ * `sharedData` is a platform API that helps sharing JS Objects across processes.
+ * We use it in order to communicate to the content process which targets and resources
+ * should be observed. Content processes read this data only once, as soon as they are created.
+ * It isn't used beyond this point. Content processes are not going to update it.
+ * We will notify about changes in observed targets and resources for already running
+ * processes by some other means. (Via JS Window Actor queries "DevTools:(un)watch(Resources|Target)")
+ * This means that only this module will update the "DevTools:watchedPerWatcher" value.
+ * From the parent process, we should be going through this module to fetch the data,
+ * while from the content process, we will read `sharedData` directly.
+ */
+
+var EXPORTED_SYMBOLS = ["WatcherRegistry"];
+
+const { Services } = ChromeUtils.import("resource://gre/modules/Services.jsm");
+const { ActorManagerParent } = ChromeUtils.import(
+  "resource://gre/modules/ActorManagerParent.jsm"
+);
+const { WatchedDataHelpers } = ChromeUtils.import(
+  "resource://devtools/server/actors/watcher/WatchedDataHelpers.jsm"
+);
+const { SUPPORTED_DATA } = WatchedDataHelpers;
+
+// Define the Map that will be saved in `sharedData`.
+// It is keyed by WatcherActor ID and values contains following attributes:
+// - targets: Set of strings, refering to target types to be listened to
+// - resources: Set of strings, refering to resource types to be observed
+// - browserId: Optional, if set, restrict the observation to one specific Browser Element tree.
+//              It can be a tab, a top-level window or a top-level iframe (e.g. special privileged iframe)
+//              See https://searchfox.org/mozilla-central/rev/31d8600b73dc85b4cdbabf45ac3f1a9c11700d8e/dom/chrome-webidl/BrowsingContext.webidl#114-121
+//              for more information.
+// - connectionPrefix: The DevToolsConnection prefix of the watcher actor. Used to compute new actor ID in the content processes.
+//
+// Unfortunately, `sharedData` is subject to race condition and may have side effect
+// when read/written from multiple places in the same process,
+// which is why this map should be considered as the single source of truth.
+const watchedDataByWatcherActor = new Map();
+
+// In parallel to the previous map, keep all the WatcherActor keyed by the same WatcherActor ID,
+// the WatcherActor ID. We don't (and can't) propagate the WatcherActor instances to the content
+// processes, but still would like to match them by their ID.
+const watcherActors = new Map();
+
+// Name of the attribute into which we save this Map in `sharedData` object.
+const SHARED_DATA_KEY_NAME = "DevTools:watchedPerWatcher";
+
+/**
+ * Use `sharedData` to allow processes, early during their creation,
+ * to know which resources should be listened to. This will be read
+ * from the Target actor, when it gets created early during process start,
+ * in order to start listening to the expected resource types.
+ */
+function persistMapToSharedData() {
+  Services.ppmm.sharedData.set(SHARED_DATA_KEY_NAME, watchedDataByWatcherActor);
+  // Request to immediately flush the data to the content processes in order to prevent
+  // races (bug 1644649). Otherwise content process may have outdated sharedData
+  // and try to create targets for Watcher actor that already stopped watching for targets.
+  Services.ppmm.sharedData.flush();
+}
+
+const WatcherRegistry = {
+  /**
+   * Tells if a given watcher currently watches for a given target type.
+   *
+   * @param WatcherActor watcher
+   *               The WatcherActor which should be listening.
+   * @param string targetType
+   *               The new target type to query.
+   * @return boolean
+   *         Returns true if already watching.
+   */
+  isWatchingTargets(watcher, targetType) {
+    const watchedData = this.getWatchedData(watcher);
+    return watchedData && watchedData.targets.includes(targetType);
+  },
+
+  /**
+   * Retrieve the data saved into `sharedData` that is used to know
+   * about which type of targets and resources we care listening about.
+   * `watchedDataByWatcherActor` is saved into `sharedData` after each mutation,
+   * but `watchedDataByWatcherActor` is the source of truth.
+   *
+   * @param WatcherActor watcher
+   *               The related WatcherActor which starts/stops observing.
+   * @param object options (optional)
+   *               A dictionary object with `createData` boolean attribute.
+   *               If this attribute is set to true, we create the data structure in the Map
+   *               if none exists for this prefix.
+   */
+  getWatchedData(watcher, { createData = false } = {}) {
+    // Use WatcherActor ID as a key as we may have multiple clients willing to watch for targets.
+    // For example, a Browser Toolbox debugging everything and a Content Toolbox debugging
+    // just one tab. We might also have multiple watchers, on the same connection when using about:debugging.
+    const watcherActorID = watcher.actorID;
+    let watchedData = watchedDataByWatcherActor.get(watcherActorID);
+    if (!watchedData && createData) {
+      watchedData = {
+        // The Browser ID will be helpful to identify which BrowsingContext should be considered
+        // when running code in the content process. Browser ID, compared to BrowsingContext ID won't change
+        // if we navigate to the parent process or if a new BrowsingContext is used for the <browser> element
+        // we are currently inspecting.
+        browserId: watcher.browserId,
+        // The DevToolsServerConnection prefix will be used to compute actor IDs created in the content process
+        connectionPrefix: watcher.conn.prefix,
+      };
+      // Define empty default array for all data
+      for (const name of Object.values(SUPPORTED_DATA)) {
+        watchedData[name] = [];
+      }
+      watchedDataByWatcherActor.set(watcherActorID, watchedData);
+      watcherActors.set(watcherActorID, watcher);
+    }
+    return watchedData;
+  },
+
+  /**
+   * Given a Watcher Actor ID, return the related Watcher Actor instance.
+   *
+   * @param String actorID
+   *        The Watcher Actor ID to search for.
+   * @return WatcherActor
+   *         The Watcher Actor instance.
+   */
+  getWatcher(actorID) {
+    return watcherActors.get(actorID);
+  },
+
+  /**
+   * Notify that a given watcher added an entry in a given data type.
+   *
+   * @param WatcherActor watcher
+   *               The WatcherActor which starts observing.
+   * @param string type
+   *               The type of data to be added
+   * @param Array<Object> entries
+   *               The values to be added to this type of data
+   */
+  addWatcherDataEntry(watcher, type, entries) {
+    const watchedData = this.getWatchedData(watcher, {
+      createData: true,
+    });
+
+    if (!(type in watchedData)) {
+      throw new Error(`Unsupported watcher data type: ${type}`);
+    }
+
+    WatchedDataHelpers.addWatchedDataEntry(watchedData, type, entries);
+
+    // Register the JS Window Actor the first time we start watching for something (e.g. resource, target, …).
+    registerJSWindowActor();
+
+    persistMapToSharedData();
+  },
+
+  /**
+   * Notify that a given watcher removed an entry in a given data type.
+   *
+   * See `addWatcherDataEntry` for argument definition.
+   *
+   * @return boolean
+   *         True if we such entry was already registered, for this watcher actor.
+   */
+  removeWatcherDataEntry(watcher, type, entries) {
+    const watchedData = this.getWatchedData(watcher);
+    if (!watchedData) {
+      return false;
+    }
+
+    if (!(type in watchedData)) {
+      throw new Error(`Unsupported watcher data type: ${type}`);
+    }
+
+    if (
+      !WatchedDataHelpers.removeWatchedDataEntry(watchedData, type, entries)
+    ) {
+      return false;
+    }
+
+    const isWatchingSomething = Object.values(SUPPORTED_DATA).some(
+      dataType => watchedData[dataType].length > 0
+    );
+    if (!isWatchingSomething) {
+      watchedDataByWatcherActor.delete(watcher.actorID);
+      watcherActors.delete(watcher.actorID);
+    }
+
+    persistMapToSharedData();
+
+    return true;
+  },
+
+  /**
+   * Cleanup everything about a given watcher actor.
+   * Remove it from any registry so that we stop interacting with it.
+   *
+   * The watcher would be automatically unregistered from removeWatcherEntry,
+   * if we remove all entries. But we aren't removing all breakpoints.
+   * So here, we force clearing any reference to the watcher actor when it destroys.
+   */
+  unregisterWatcher(watcher) {
+    watchedDataByWatcherActor.delete(watcher.actorID);
+    watcherActors.delete(watcher.actorID);
+  },
+
+  /**
+   * Notify that a given watcher starts observing a new target type.
+   *
+   * @param WatcherActor watcher
+   *               The WatcherActor which starts observing.
+   * @param string targetType
+   *               The new target type to start listening to.
+   */
+  watchTargets(watcher, targetType) {
+    this.addWatcherDataEntry(watcher, SUPPORTED_DATA.TARGETS, [targetType]);
+  },
+
+  /**
+   * Notify that a given watcher stops observing a given target type.
+   *
+   * See `watchTargets` for argument definition.
+   *
+   * @return boolean
+   *         True if we were watching for this target type, for this watcher actor.
+   */
+  unwatchTargets(watcher, targetType) {
+    return this.removeWatcherDataEntry(watcher, SUPPORTED_DATA.TARGETS, [
+      targetType,
+    ]);
+  },
+
+  /**
+   * Notify that a given watcher starts observing new resource types.
+   *
+   * @param WatcherActor watcher
+   *               The WatcherActor which starts observing.
+   * @param Array<string> resourceTypes
+   *               The new resource types to start listening to.
+   */
+  watchResources(watcher, resourceTypes) {
+    this.addWatcherDataEntry(watcher, SUPPORTED_DATA.RESOURCES, resourceTypes);
+  },
+
+  /**
+   * Notify that a given watcher stops observing given resource types.
+   *
+   * See `watchResources` for argument definition.
+   *
+   * @return boolean
+   *         True if we were watching for this resource type, for this watcher actor.
+   */
+  unwatchResources(watcher, resourceTypes) {
+    return this.removeWatcherDataEntry(
+      watcher,
+      SUPPORTED_DATA.RESOURCES,
+      resourceTypes
+    );
+  },
+
+  /**
+   * Unregister the JS Window Actor if there is no more DevTools code observing any target/resource.
+   */
+  maybeUnregisteringJSWindowActor() {
+    if (watchedDataByWatcherActor.size == 0) {
+      unregisterJSWindowActor();
+    }
+  },
+};
+
+// Boolean flag to know if the DevToolsFrame JS Window Actor is currently registered
+let isJSWindowActorRegistered = false;
+
+/**
+ * Register the JSWindowActor pair "DevToolsFrame".
+ *
+ * We should call this method before we try to use this JS Window Actor from the parent process
+ * (via `WindowGlobal.getActor("DevToolsFrame")` or `WindowGlobal.getActor("DevToolsWorker")`).
+ * Also, registering it will automatically force spawing the content process JSWindow Actor
+ * anytime a new document is opened (via DOMWindowCreated event).
+ */
+
+const JSWindowActorsConfig = {
+  DevToolsFrame: {
+    parent: {
+      moduleURI:
+        "resource://devtools/server/connectors/js-window-actor/DevToolsFrameParent.jsm",
+    },
+    child: {
+      moduleURI:
+        "resource://devtools/server/connectors/js-window-actor/DevToolsFrameChild.jsm",
+      events: {
+        DOMWindowCreated: {},
+      },
+    },
+    allFrames: true,
+  },
+  DevToolsWorker: {
+    parent: {
+      moduleURI:
+        "resource://devtools/server/connectors/js-window-actor/DevToolsWorkerParent.jsm",
+    },
+    child: {
+      moduleURI:
+        "resource://devtools/server/connectors/js-window-actor/DevToolsWorkerChild.jsm",
+      events: {
+        DOMWindowCreated: {},
+      },
+    },
+    allFrames: true,
+  },
+};
+
+function registerJSWindowActor() {
+  if (isJSWindowActorRegistered) {
+    return;
+  }
+  isJSWindowActorRegistered = true;
+  ActorManagerParent.addJSWindowActors(JSWindowActorsConfig);
+}
+
+function unregisterJSWindowActor() {
+  if (!isJSWindowActorRegistered) {
+    return;
+  }
+  isJSWindowActorRegistered = false;
+
+  for (const JSWindowActorName of Object.keys(JSWindowActorsConfig)) {
+    // ActorManagerParent doesn't expose a "removeActors" method, but it would be equivalent to that:
+    ChromeUtils.unregisterWindowActor(JSWindowActorName);
+  }
+}