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diff --git a/toolkit/components/extensions/WebNavigationFrames.sys.mjs b/toolkit/components/extensions/WebNavigationFrames.sys.mjs
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+++ b/toolkit/components/extensions/WebNavigationFrames.sys.mjs
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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/. */
+
+/**
+ * The FrameDetail object which represents a frame in WebExtensions APIs.
+ *
+ * @typedef  {object}  FrameDetail
+ * @inner
+ * @property {number}  frameId        - Represents the numeric id which identify the frame in its tab.
+ * @property {number}  parentFrameId  - Represents the numeric id which identify the parent frame.
+ * @property {string}  url            - Represents the current location URL loaded in the frame.
+ * @property {boolean} errorOccurred  - Indicates whether an error is occurred during the last load
+ *                                      happened on this frame (NOT YET SUPPORTED).
+ */
+
+/**
+ * Returns the frame ID of the given window. If the window is the
+ * top-level content window, its frame ID is 0. Otherwise, its frame ID
+ * is its outer window ID.
+ *
+ * @param {Window|BrowsingContext} bc - The window to retrieve the frame ID for.
+ * @returns {number}
+ */
+function getFrameId(bc) {
+  if (!BrowsingContext.isInstance(bc)) {
+    bc = bc.browsingContext;
+  }
+  return bc.parent ? bc.id : 0;
+}
+
+/**
+ * Returns the frame ID of the given window's parent.
+ *
+ * @param {Window|BrowsingContext} bc - The window to retrieve the parent frame ID for.
+ * @returns {number}
+ */
+function getParentFrameId(bc) {
+  if (!BrowsingContext.isInstance(bc)) {
+    bc = bc.browsingContext;
+  }
+  return bc.parent ? getFrameId(bc.parent) : -1;
+}
+
+/**
+ * Convert a BrowsingContext into internal FrameDetail json.
+ *
+ * @param {BrowsingContext} bc
+ * @returns {FrameDetail}
+ */
+function getFrameDetail(bc) {
+  return {
+    frameId: getFrameId(bc),
+    parentFrameId: getParentFrameId(bc),
+    url: bc.currentURI?.spec,
+  };
+}
+
+export var WebNavigationFrames = {
+  getFrame(bc, frameId) {
+    // frameId 0 means the top-level frame; anything else is a child frame.
+    let frame = BrowsingContext.get(frameId || bc.id);
+    if (frame && frame.top === bc) {
+      return getFrameDetail(frame);
+    }
+    return null;
+  },
+
+  getFrameId,
+  getParentFrameId,
+
+  getAllFrames(bc) {
+    let frames = [];
+
+    // Recursively walk the BC tree, find all frames.
+    function visit(bc) {
+      frames.push(bc);
+      bc.children.forEach(visit);
+    }
+    visit(bc);
+    return frames.map(getFrameDetail);
+  },
+
+  getFromWindow(target) {
+    if (Window.isInstance(target)) {
+      return getFrameId(BrowsingContext.getFromWindow(target));
+    }
+    return -1;
+  },
+};