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diff --git a/browser/modules/BrowserUIUtils.sys.mjs b/browser/modules/BrowserUIUtils.sys.mjs
new file mode 100644
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+++ b/browser/modules/BrowserUIUtils.sys.mjs
@@ -0,0 +1,180 @@
+/* -*- mode: js; indent-tabs-mode: nil; js-indent-level: 2 -*- */
+/* 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/. */
+
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
+
+const lazy = {};
+
+XPCOMUtils.defineLazyPreferenceGetter(
+  lazy,
+  "trimHttps",
+  "browser.urlbar.trimHttps"
+);
+export var BrowserUIUtils = {
+  /**
+   * Check whether a page can be considered as 'empty', that its URI
+   * reflects its origin, and that if it's loaded in a tab, that tab
+   * could be considered 'empty' (e.g. like the result of opening
+   * a 'blank' new tab).
+   *
+   * We have to do more than just check the URI, because especially
+   * for things like about:blank, it is possible that the opener or
+   * some other page has control over the contents of the page.
+   *
+   * @param {Browser} browser
+   *        The browser whose page we're checking.
+   * @param {nsIURI} [uri]
+   *        The URI against which we're checking (the browser's currentURI
+   *        if omitted).
+   *
+   * @return {boolean} false if the page was opened by or is controlled by
+   *         arbitrary web content, unless that content corresponds with the URI.
+   *         true if the page is blank and controlled by a principal matching
+   *         that URI (or the system principal if the principal has no URI)
+   */
+  checkEmptyPageOrigin(browser, uri = browser.currentURI) {
+    // If another page opened this page with e.g. window.open, this page might
+    // be controlled by its opener.
+    if (browser.hasContentOpener) {
+      return false;
+    }
+    let contentPrincipal = browser.contentPrincipal;
+    // Not all principals have URIs...
+    // There are two special-cases involving about:blank. One is where
+    // the user has manually loaded it and it got created with a null
+    // principal. The other involves the case where we load
+    // some other empty page in a browser and the current page is the
+    // initial about:blank page (which has that as its principal, not
+    // just URI in which case it could be web-based). Especially in
+    // e10s, we need to tackle that case specifically to avoid race
+    // conditions when updating the URL bar.
+    //
+    // Note that we check the documentURI here, since the currentURI on
+    // the browser might have been set by SessionStore in order to
+    // support switch-to-tab without having actually loaded the content
+    // yet.
+    let uriToCheck = browser.documentURI || uri;
+    if (
+      (uriToCheck.spec == "about:blank" && contentPrincipal.isNullPrincipal) ||
+      contentPrincipal.spec == "about:blank"
+    ) {
+      return true;
+    }
+    if (contentPrincipal.isContentPrincipal) {
+      return contentPrincipal.equalsURI(uri);
+    }
+    // ... so for those that don't have them, enforce that the page has the
+    // system principal (this matches e.g. on about:newtab).
+    return contentPrincipal.isSystemPrincipal;
+  },
+
+  /**
+   * Generate a document fragment for a localized string that has DOM
+   * node replacements. This avoids using getFormattedString followed
+   * by assigning to innerHTML. Fluent can probably replace this when
+   * it is in use everywhere.
+   *
+   * @param {Document} doc
+   * @param {String}   msg
+   *                   The string to put replacements in. Fetch from
+   *                   a stringbundle using getString or GetStringFromName,
+   *                   or even an inserted dtd string.
+   * @param {Node|String} nodesOrStrings
+   *                   The replacement items. Can be a mix of Nodes
+   *                   and Strings. However, for correct behaviour, the
+   *                   number of items provided needs to exactly match
+   *                   the number of replacement strings in the l10n string.
+   * @returns {DocumentFragment}
+   *                   A document fragment. In the trivial case (no
+   *                   replacements), this will simply be a fragment with 1
+   *                   child, a text node containing the localized string.
+   */
+  getLocalizedFragment(doc, msg, ...nodesOrStrings) {
+    // Ensure replacement points are indexed:
+    for (let i = 1; i <= nodesOrStrings.length; i++) {
+      if (!msg.includes("%" + i + "$S")) {
+        msg = msg.replace(/%S/, "%" + i + "$S");
+      }
+    }
+    let numberOfInsertionPoints = msg.match(/%\d+\$S/g).length;
+    if (numberOfInsertionPoints != nodesOrStrings.length) {
+      console.error(
+        `Message has ${numberOfInsertionPoints} insertion points, ` +
+          `but got ${nodesOrStrings.length} replacement parameters!`
+      );
+    }
+
+    let fragment = doc.createDocumentFragment();
+    let parts = [msg];
+    let insertionPoint = 1;
+    for (let replacement of nodesOrStrings) {
+      let insertionString = "%" + insertionPoint++ + "$S";
+      let partIndex = parts.findIndex(
+        part => typeof part == "string" && part.includes(insertionString)
+      );
+      if (partIndex == -1) {
+        fragment.appendChild(doc.createTextNode(msg));
+        return fragment;
+      }
+
+      if (typeof replacement == "string") {
+        parts[partIndex] = parts[partIndex].replace(
+          insertionString,
+          replacement
+        );
+      } else {
+        let [firstBit, lastBit] = parts[partIndex].split(insertionString);
+        parts.splice(partIndex, 1, firstBit, replacement, lastBit);
+      }
+    }
+
+    // Put everything in a document fragment:
+    for (let part of parts) {
+      if (typeof part == "string") {
+        if (part) {
+          fragment.appendChild(doc.createTextNode(part));
+        }
+      } else {
+        fragment.appendChild(part);
+      }
+    }
+    return fragment;
+  },
+
+  removeSingleTrailingSlashFromURL(aURL) {
+    // remove single trailing slash for http/https/ftp URLs
+    return aURL.replace(/^((?:http|https|ftp):\/\/[^/]+)\/$/, "$1");
+  },
+
+  get trimURLProtocol() {
+    return lazy.trimHttps ? "https://" : "http://";
+  },
+
+  /**
+   * Returns a URL which has been trimmed by removing 'http://' or 'https://',
+   * when the pref 'trimHttps' is set to true, and any trailing slash
+   * (in http/https/ftp urls). Note that a trimmed url may not load the same
+   * page as the original url, so before loading it, it must be passed through
+   * URIFixup, to check trimming doesn't change its destination. We don't run
+   * the URIFixup check here, because trimURL is in the page load path
+   * (see onLocationChange), so it must be fast and simple.
+   *
+   * @param {string} aURL The URL to trim.
+   * @returns {string} The trimmed string.
+   */
+  trimURL(aURL) {
+    let url = this.removeSingleTrailingSlashFromURL(aURL);
+    return url.startsWith(this.trimURLProtocol)
+      ? url.substring(this.trimURLProtocol.length)
+      : url;
+  },
+};
+
+XPCOMUtils.defineLazyPreferenceGetter(
+  BrowserUIUtils,
+  "quitShortcutDisabled",
+  "browser.quitShortcut.disabled",
+  false
+);