From d8bbc7858622b6d9c278469aab701ca0b609cddf Mon Sep 17 00:00:00 2001
From: Daniel Baumann <daniel.baumann@progress-linux.org>
Date: Wed, 15 May 2024 05:35:49 +0200
Subject: Merging upstream version 126.0.

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
---
 browser/base/content/aboutDialog-appUpdater.js     |   2 +-
 browser/base/content/aboutDialog.xhtml             |   2 +-
 browser/base/content/appmenu-viewcache.inc.xhtml   |  35 +-
 browser/base/content/browser-a11yUtils.js          |   8 +-
 browser/base/content/browser-addons.js             |  33 +-
 browser/base/content/browser-allTabsMenu.js        |   6 +-
 browser/base/content/browser-box.inc.xhtml         |   1 +
 browser/base/content/browser-captivePortal.js      |   2 +-
 browser/base/content/browser-commands.js           | 576 +++++++++++++
 browser/base/content/browser-context.inc           |   2 +-
 browser/base/content/browser-ctrlTab.js            |   4 +-
 .../content/browser-data-submission-info-bar.js    |   2 +-
 .../content/browser-fullScreenAndPointerLock.js    |  43 +-
 browser/base/content/browser-gestureSupport.js     |  10 +-
 browser/base/content/browser-menubar.inc           |   5 +-
 browser/base/content/browser-pageActions.js        |   2 +-
 browser/base/content/browser-places.js             |  14 +-
 browser/base/content/browser-sets.inc              |  38 +-
 browser/base/content/browser-sidebar.js            | 674 ----------------
 browser/base/content/browser-siteIdentity.js       |  14 +-
 .../base/content/browser-sitePermissionPanel.js    |   4 +-
 browser/base/content/browser-siteProtections.js    |  30 +-
 browser/base/content/browser-sync.js               |  10 +-
 browser/base/content/browser-tabsintitlebar.js     |   2 +-
 browser/base/content/browser-thumbnails.js         |   4 +-
 browser/base/content/browser-toolbarKeyNav.js      |   2 +-
 browser/base/content/browser.css                   |  13 +-
 browser/base/content/browser.js                    | 894 ++-------------------
 browser/base/content/browser.js.globals            |  22 +-
 browser/base/content/browser.xhtml                 |   6 +-
 browser/base/content/contentTheme.js               |  21 +-
 browser/base/content/main-popupset.inc.xhtml       |  26 +-
 browser/base/content/navigator-toolbox.inc.xhtml   |   9 +-
 browser/base/content/nsContextMenu.js              |  58 +-
 browser/base/content/pageinfo/pageInfo.js          |  40 +-
 browser/base/content/pageinfo/pageInfo.xhtml       |   4 +
 browser/base/content/pageinfo/permissions.js       |   2 +-
 browser/base/content/pageinfo/security.js          |   2 +-
 browser/base/content/sanitizeDialog.js             |  23 +-
 browser/base/content/tabbrowser-tab.js             |  26 +-
 browser/base/content/tabbrowser-tabs.js            |  54 +-
 browser/base/content/tabbrowser.js                 |  83 +-
 .../content/test/about/browser_aboutCertError.js   |   4 +-
 .../alerts/browser_notification_open_settings.js   |   2 +-
 browser/base/content/test/alerts/head.js           |   2 +-
 .../browser_captivePortal_certErrorUI.js           |   2 +-
 browser/base/content/test/contextMenu/browser.toml |   4 +-
 .../test/contextMenu/browser_contextmenu.js        |   2 +-
 .../contextMenu/browser_contextmenu_badiframe.js   |   2 +-
 .../browser_contextmenu_save_blocked.js            |   2 +-
 .../browser_strip_on_share_nested_link.js          | 162 ++++
 .../content/test/contextMenu/contextmenu_common.js |   4 +-
 .../test/contextMenu/subtst_contextmenu.html       |   4 +-
 .../contextMenu/subtst_contextmenu_webext.html     |   2 +-
 .../browser_favicon_change_not_in_document.js      |   8 +-
 .../content/test/favicons/browser_favicon_load.js  |   2 +-
 .../test/favicons/browser_favicon_nostore.js       |  13 +-
 .../test/favicons/browser_favicon_referer.js       |   4 +-
 .../test/favicons/browser_missing_favicon.js       |   2 +-
 .../base/content/test/forms/browser_selectpopup.js |   4 +-
 .../test/forms/browser_selectpopup_colors.js       |   2 +-
 .../content/test/forms/browser_selectpopup_dir.js  |   2 +-
 .../test/forms/browser_selectpopup_large.js        |   2 +-
 .../test/forms/browser_selectpopup_minFontSize.js  |   2 +-
 .../forms/browser_selectpopup_text_transform.js    |   2 +-
 .../test/forms/browser_selectpopup_user_input.js   |   2 +-
 .../test/forms/browser_selectpopup_width.js        |   2 +-
 .../test/forms/browser_selectpopup_xhtml.js        |   2 +-
 .../fullscreen/browser_fullscreen_context_menu.js  |   4 +-
 .../fullscreen/browser_fullscreen_window_focus.js  |   2 +-
 browser/base/content/test/fullscreen/head.js       |   2 +-
 browser/base/content/test/general/browser.toml     |  15 +-
 .../content/test/general/browser_accesskeys.js     |   4 +-
 .../test/general/browser_alltabslistener.js        |  15 +-
 .../browser_beforeunload_duplicate_dialogs.js      |   4 +-
 .../base/content/test/general/browser_bug356571.js |   2 +-
 .../base/content/test/general/browser_bug417483.js |   2 +-
 .../base/content/test/general/browser_bug537013.js |   2 +-
 .../base/content/test/general/browser_bug565575.js |   2 +-
 .../base/content/test/general/browser_bug567306.js |   2 +-
 .../base/content/test/general/browser_bug609700.js |   6 +-
 .../base/content/test/general/browser_bug623893.js |   2 +-
 .../base/content/test/general/browser_bug676619.js |  12 +-
 .../base/content/test/general/browser_bug734076.js |   6 +-
 .../test/general/browser_bug763468_perwindowpb.js  |   2 +-
 .../test/general/browser_bug767836_perwindowpb.js  |   2 +-
 .../base/content/test/general/browser_bug817947.js |   2 +-
 .../base/content/test/general/browser_clipboard.js |   6 +-
 .../test/general/browser_clipboard_pastefile.js    |   4 +-
 .../browser_domFullscreen_fullscreenMode.js        |   6 +-
 .../test/general/browser_double_close_tab.js       |   2 +-
 .../content/test/general/browser_focusonkeydown.js |   2 +-
 .../test/general/browser_fullscreen-window-open.js |   8 +-
 ...rowser_invalid_uri_back_forward_manipulation.js |   2 +-
 .../content/test/general/browser_newWindowDrop.js  |   2 +-
 .../content/test/general/browser_plainTextLinks.js |   2 +-
 .../test/general/browser_private_no_prompt.js      |   4 +-
 .../test/general/browser_remoteTroubleshoot.js     |   4 +-
 .../test/general/browser_save_link-perwindowpb.js  |   4 +-
 .../browser_save_link_when_window_navigates.js     |   6 +-
 .../browser_save_private_link_perwindowpb.js       |   6 +-
 .../content/test/general/browser_save_video.js     |   2 +-
 .../base/content/test/general/browser_tabfocus.js  |   4 +-
 .../content/test/general/browser_tabs_owner.js     |   6 +-
 .../general/browser_viewSourceInTabOnViewSource.js |   6 +-
 .../content/test/general/browser_zbug569342.js     |   2 +-
 .../base/content/test/general/download_page.html   |  12 +-
 browser/base/content/test/general/head.js          |   4 +-
 browser/base/content/test/general/video.ogg        | Bin 285310 -> 0 bytes
 browser/base/content/test/general/video.webm       | Bin 0 -> 222879 bytes
 browser/base/content/test/general/web_video.html   |   2 +-
 browser/base/content/test/general/web_video1.ogv   | Bin 28942 -> 0 bytes
 .../content/test/general/web_video1.ogv^headers^   |   3 -
 browser/base/content/test/general/web_video1.webm  | Bin 0 -> 17555 bytes
 .../content/test/general/web_video1.webm^headers^  |   3 +
 .../browser_historySwipeAnimation.js               |   2 +-
 .../test/keyboard/browser_toolbarButtonKeyPress.js |  93 +--
 .../content/test/metaTags/browser_bad_meta_tags.js |  11 +-
 .../content/test/metaTags/browser_meta_tags.js     |   8 +-
 .../outOfProcess/browser_basic_outofprocess.js     |   9 +-
 browser/base/content/test/pageActions/head.js      |   2 +-
 browser/base/content/test/pageinfo/browser.toml    |   2 +-
 .../browser_pageinfo_firstPartyIsolation.js        |   2 +-
 .../test/pageinfo/browser_pageinfo_iframe_media.js |   2 +-
 .../test/pageinfo/browser_pageinfo_image_info.js   |   2 +-
 .../test/pageinfo/browser_pageinfo_images.js       |   4 +-
 .../test/pageinfo/browser_pageinfo_permissions.js  |  12 +-
 .../content/test/pageinfo/browser_pageinfo_rtl.js  |  25 +-
 .../test/pageinfo/browser_pageinfo_security.js     |  21 +-
 .../pageinfo/browser_pageinfo_separate_private.js  |   4 +-
 .../test/pageinfo/browser_pageinfo_svg_image.js    |   2 +-
 browser/base/content/test/pageinfo/image.html      |   2 +-
 .../performance/StartupContentSubframe.sys.mjs     |   2 +-
 .../test/performance/browser_preferences_usage.js  |   2 +-
 .../test/performance/browser_startup_content.js    |  10 +-
 .../performance/browser_startup_mainthreadio.js    |   1 +
 .../content/test/performance/browser_tabdetach.js  |   4 +-
 .../content/test/performance/browser_tabopen.js    |   2 +-
 .../test/performance/browser_tabopen_squeeze.js    |   2 +-
 .../browser_tabstrip_overflow_underflow.js         |   4 +-
 .../content/test/performance/browser_tabswitch.js  |  14 +-
 .../test/performance/browser_windowclose.js        |   2 +-
 .../content/test/performance/browser_windowopen.js |   2 +-
 browser/base/content/test/performance/head.js      |  21 +-
 .../test/permissions/browser_autoplay_blocked.js   |   2 +-
 .../browser_canvas_fingerprinting_resistance.js    |   8 +-
 .../permissions/browser_site_scoped_permissions.js |  89 +-
 .../browser_temporary_permissions_navigation.js    |   2 +-
 browser/base/content/test/plugins/head.js          |   4 +-
 .../browser_popupNotification.js                   |  22 +-
 .../browser_popupNotification_2.js                 |  20 +-
 .../browser_popupNotification_3.js                 |  12 +-
 .../browser_popupNotification_4.js                 |  10 +-
 .../browser_popupNotification_5.js                 |  14 +-
 .../browser_popupNotification_accesskey.js         |   2 +-
 .../browser_popupNotification_keyboard.js          |   6 +-
 .../browser_popupNotification_no_anchors.js        |   6 +-
 .../browser_popupNotification_security_delay.js    |   2 +-
 .../test/popups/browser_popup_close_main_window.js |   6 +-
 .../test/protectionsUI/browser_protectionsUI.js    |   2 +-
 .../browser_protectionsUI_cookies_subview.js       |   2 +-
 .../protectionsUI/browser_protectionsUI_fetch.js   |   2 +-
 .../browser_protectionsUI_info_message.js          |   4 +-
 browser/base/content/test/referrer/head.js         |   2 +-
 .../sanitize/browser_cookiePermission_aboutURL.js  |   4 +-
 .../test/sanitize/browser_sanitize-timespans.js    |   2 +-
 .../test/sanitize/browser_sanitize-timespans_v2.js |   2 +-
 .../test/sanitize/browser_sanitizeDialog_v2.js     |   4 +-
 .../browser_sanitizeDialog_v2_dataSizes.js         |  65 +-
 browser/base/content/test/sanitize/head.js         |   4 +-
 .../content/test/sidebar/browser_sidebar_move.js   |  14 +-
 .../siteIdentity/browser_identityBlock_focus.js    |   2 +-
 .../browser_identityPopup_clearSiteData.js         |   2 +-
 .../siteIdentity/browser_navigation_failures.js    |   4 +-
 .../browser_secure_transport_insecure_scheme.js    |   8 +-
 browser/base/content/test/siteIdentity/head.js     |  12 +-
 .../test/static/browser_all_files_referenced.js    |  26 +-
 .../content/test/static/browser_parsable_css.js    |  14 +-
 .../content/test/static/browser_parsable_script.js |   8 +-
 browser/base/content/test/static/head.js           |   2 +-
 .../test/sync/browser_contextmenu_sendtab.js       |   2 +-
 .../content/test/sync/browser_fxa_web_channel.js   |  16 +-
 browser/base/content/test/sync/browser_sync.js     |   4 +-
 .../browser_abort_when_in_modal_state.js           |   8 +-
 .../tabPrompts/browser_auth_spoofing_protection.js |   4 +-
 .../test/tabPrompts/browser_contentOrigins.js      |   6 -
 .../test/tabPrompts/browser_windowPrompt.js        |  16 -
 .../tabcrashed/browser_aboutRestartRequired.toml   |  10 +-
 .../browser_aboutRestartRequired_noForkServer.toml |   8 +
 browser/base/content/test/tabcrashed/head.js       |   8 +-
 browser/base/content/test/tabs/browser.toml        |   2 +
 ...low_process_switches_despite_related_browser.js |   2 +-
 .../base/content/test/tabs/browser_audioTabIcon.js |   4 +-
 .../browser_e10s_about_page_triggeringprincipal.js |   4 +-
 .../test/tabs/browser_e10s_about_process.js        |   4 +-
 .../content/test/tabs/browser_lastSeenActive.js    | 260 ++++++
 .../test/tabs/browser_lazy_tab_browser_events.js   |  14 +-
 ...e_and_url_prefilled_normal_page_blank_target.js |   4 +-
 ...itle_and_url_prefilled_normal_page_by_script.js |   2 +-
 ...itle_and_url_prefilled_normal_page_no_target.js |   2 +-
 ...e_and_url_prefilled_normal_page_other_target.js |   4 +-
 .../tabs/browser_long_data_url_label_truncation.js |   2 +-
 ...tiselect_tabs_move_to_new_window_contextmenu.js |  16 +-
 .../browser_new_tab_bookmarks_toolbar_height.js    |   4 +-
 ...wser_new_tab_in_privilegedabout_process_pref.js |   2 +-
 .../base/content/test/tabs/browser_new_tab_url.js  |   4 +-
 ...wser_open_newtab_start_observer_notification.js |   4 +-
 .../tabs/browser_pinnedTabs_closeByKeyboard.js     |   4 +-
 .../tabs/browser_privilegedmozilla_process_pref.js |   2 +-
 .../content/test/tabs/browser_removeTabs_order.js  |   2 +-
 .../tabs/browser_tab_label_picture_in_picture.js   |   2 +-
 .../test/tabs/browser_tab_manager_visibility.js    |   4 +-
 .../base/content/test/tabs/browser_tab_preview.js  | 171 +++-
 .../base/content/test/tabs/browser_tab_tooltips.js |   2 +-
 .../content/test/tabs/browser_tabswitch_select.js  |   4 +-
 .../test/tabs/browser_tabswitch_updatecommands.js  |   2 +-
 ...owser_viewsource_of_data_URI_in_file_process.js |   2 +-
 .../test/tabs/browser_window_open_modifiers.js     |   2 +-
 .../common_link_in_tab_title_and_url_prefilled.js  |   2 +-
 .../browser_extension_update_background.js         |   6 +-
 ...browser_extension_update_background_noprompt.js |   2 +-
 .../test/webextensions/browser_legacy_webext.xpi   | Bin 4243 -> 362 bytes
 .../browser_permissions_installTrigger.js          |   6 +
 .../browser_permissions_local_file.js              |  13 +
 .../browser_permissions_mozAddonManager.js         |  13 +
 .../browser_permissions_pointerevent.js            |   6 +-
 .../browser_update_checkForUpdates.js              |   2 +-
 .../webextensions/browser_webext_nopermissions.xpi | Bin 4273 -> 7500 bytes
 .../webextensions/browser_webext_permissions.xpi   | Bin 16602 -> 19923 bytes
 .../test/webextensions/browser_webext_update1.xpi  | Bin 4271 -> 326 bytes
 .../test/webextensions/browser_webext_update2.xpi  | Bin 4291 -> 343 bytes
 .../webextensions/browser_webext_update_icon1.xpi  | Bin 16545 -> 12581 bytes
 .../webextensions/browser_webext_update_icon2.xpi  | Bin 16564 -> 12599 bytes
 .../webextensions/browser_webext_update_perms1.xpi | Bin 4273 -> 320 bytes
 .../webextensions/browser_webext_update_perms2.xpi | Bin 4282 -> 331 bytes
 browser/base/content/test/webextensions/head.js    |   2 +-
 .../webrtc/browser_devices_get_user_media_anim.js  |   4 +-
 .../browser_devices_get_user_media_by_device_id.js |   2 +-
 .../webrtc/browser_devices_get_user_media_grace.js |   6 +-
 .../webrtc/browser_devices_select_audio_output.js  |   2 +-
 .../content/test/webrtc/browser_webrtc_hooks.js    |  28 +-
 browser/base/content/test/webrtc/head.js           |   6 +-
 browser/base/content/test/zoom/browser.toml        |   2 +-
 .../test/zoom/browser_sitespecific_video_zoom.js   |   2 +-
 .../content/test/zoom/browser_zoom_commands.js     |   4 +-
 browser/base/content/test/zoom/head.js             |   8 +-
 browser/base/content/titlebar-items.inc.xhtml      |   2 +-
 browser/base/content/utilityOverlay.js             |   4 +-
 browser/base/content/webext-panels.js              |  12 +-
 browser/base/content/webrtcIndicator.js            |   2 +-
 browser/base/jar.mn                                |   2 +-
 251 files changed, 2117 insertions(+), 2532 deletions(-)
 create mode 100644 browser/base/content/browser-commands.js
 delete mode 100644 browser/base/content/browser-sidebar.js
 create mode 100644 browser/base/content/test/contextMenu/browser_strip_on_share_nested_link.js
 delete mode 100644 browser/base/content/test/general/video.ogg
 create mode 100644 browser/base/content/test/general/video.webm
 delete mode 100644 browser/base/content/test/general/web_video1.ogv
 delete mode 100644 browser/base/content/test/general/web_video1.ogv^headers^
 create mode 100644 browser/base/content/test/general/web_video1.webm
 create mode 100644 browser/base/content/test/general/web_video1.webm^headers^
 create mode 100644 browser/base/content/test/tabs/browser_lastSeenActive.js

(limited to 'browser/base')

diff --git a/browser/base/content/aboutDialog-appUpdater.js b/browser/base/content/aboutDialog-appUpdater.js
index 21bf83bc42..5a8cc0561b 100644
--- a/browser/base/content/aboutDialog-appUpdater.js
+++ b/browser/base/content/aboutDialog-appUpdater.js
@@ -28,7 +28,7 @@ var UPDATING_MIN_DISPLAY_TIME_MS = 1500;
 
 var gAppUpdater;
 
-function onUnload(aEvent) {
+function onUnload(_aEvent) {
   if (gAppUpdater) {
     gAppUpdater.destroy();
     gAppUpdater = null;
diff --git a/browser/base/content/aboutDialog.xhtml b/browser/base/content/aboutDialog.xhtml
index e0fcce367a..55de242415 100644
--- a/browser/base/content/aboutDialog.xhtml
+++ b/browser/base/content/aboutDialog.xhtml
@@ -138,7 +138,7 @@
             <label is="text-link" useoriginprincipal="true" href="about:credits" data-l10n-name="community-creditsLink"/>
           </description>
           <description class="text-blurb" id="contributeDesc" data-l10n-id="helpus">
-            <label is="text-link" href="https://donate.mozilla.org/?utm_source=firefox&#38;utm_medium=referral&#38;utm_campaign=firefox_about&#38;utm_content=firefox_about" data-l10n-name="helpus-donateLink"/>
+            <label is="text-link" href="https://foundation.mozilla.org/?form=firefox-about" data-l10n-name="helpus-donateLink"/>
             <label is="text-link" href="https://www.mozilla.org/contribute/?utm_source=firefox-browser&#38;utm_medium=firefox-desktop&#38;utm_campaign=about-dialog" data-l10n-name="helpus-getInvolvedLink"/>
           </description>
         </vbox>
diff --git a/browser/base/content/appmenu-viewcache.inc.xhtml b/browser/base/content/appmenu-viewcache.inc.xhtml
index 04bba182fb..6d838bd9d6 100644
--- a/browser/base/content/appmenu-viewcache.inc.xhtml
+++ b/browser/base/content/appmenu-viewcache.inc.xhtml
@@ -3,16 +3,6 @@
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 <html:template id="appMenu-viewCache">
-  <panelview id="appMenu-mainView" class="PanelUI-subView">
-    <vbox class="panel-subview-body">
-      <toolbarbutton id="appMenu-whatsnew-button"
-                     class="subviewbutton subviewbutton-iconic subviewbutton-nav"
-                     hidden="true"
-                     closemenu="none"
-                     oncommand="PanelUI.showSubView('PanelUI-whatsNew', this)"/>
-    </vbox>
-  </panelview>
-
   <!-- This is a placeholder app menu which should be replaced with the "real"
        Proton app menu before the Proton pref starts getting enabled. -->
   <panelview id="appMenu-protonMainView" class="PanelUI-subView"
@@ -134,7 +124,7 @@
 # in the middle of the fullscreen transition otherwise.
                        oncommand="
                          this.closest('panel').hidePopup();
-                         setTimeout(() => BrowserFullScreen(), 0);
+                         setTimeout(() => BrowserCommands.fullScreen(), 0);
                        "
                        tooltip="dynamic-shortcut-tooltip">
            <observes element="View:FullScreen" attribute="checked"/>
@@ -759,29 +749,6 @@
     </vbox>
   </panelview>
 
-  <panelview id="PanelUI-whatsNew" class="PanelUI-subView" mainview-with-header="true">
-    <hbox id="PanelUI-whatsNew-title" class="panel-header">
-      <html:h1>
-        <html:span data-l10n-id="whatsnew-panel-header"></html:span>
-      </html:h1>
-    </hbox>
-    <toolbarseparator/>
-    <vbox class="panel-subview-body">
-      <toolbaritem id="PanelUI-whatsNew-content"
-                   orient="vertical"
-                   smoothscroll="false">
-        <html:div id="PanelUI-whatsNew-message-container" role="document">
-          <!-- What's New messages will be rendered here -->
-        </html:div>
-      </toolbaritem>
-    </vbox>
-    <toolbarseparator/>
-    <checkbox id="panelMenu-toggleWhatsNew"
-              class="panelMenu-toggleWhatsNew-checkbox"
-              onclick="ToolbarPanelHub.toggleWhatsNewPref(event)"
-              data-l10n-id="whatsnew-panel-footer-checkbox"/>
-  </panelview>
-
   <panelview id="reset-pbm-panel" class="PanelUI-subView" role="document">
     <vbox id="reset-pbm-panel-container" role="alertdialog" aria-labelledby="reset-pbm-panel-header">
       <hbox id="reset-pbm-panel-header">
diff --git a/browser/base/content/browser-a11yUtils.js b/browser/base/content/browser-a11yUtils.js
index 9bedb9238c..935ddc6a55 100644
--- a/browser/base/content/browser-a11yUtils.js
+++ b/browser/base/content/browser-a11yUtils.js
@@ -21,6 +21,7 @@ var A11yUtils = {
    * can thus hinder rather than help users if used incorrectly.
    * Please only use this after consultation with the Mozilla accessibility
    * team.
+   * @param {object} [options]
    * @param {string} [options.id] The Fluent id of the message to announce. The
    *        ftl file must already be included in browser.xhtml. This must be
    *        specified unless a raw message is specified instead.
@@ -28,13 +29,8 @@ var A11yUtils = {
    * @param {string} [options.raw] The raw, already localized message to
    *        announce. You should generally prefer a Fluent id instead, but in
    *        rare cases, this might not be feasible.
-   * @param {Element} [options.source] The element with which the announcement
-   *        is associated. This should generally be something the user can
-   *        interact with to respond to the announcement. For example, for an
-   *        announcement indicating that Reader View is available, this should
-   *        be the Reader View button on the toolbar.
    */
-  async announce({ id = null, args = {}, raw = null, source = document } = {}) {
+  async announce({ id = null, args = {}, raw = null } = {}) {
     if ((!id && !raw) || (id && raw)) {
       throw new Error("One of raw or id must be specified.");
     }
diff --git a/browser/base/content/browser-addons.js b/browser/base/content/browser-addons.js
index 6f50745e8d..1e97428e24 100644
--- a/browser/base/content/browser-addons.js
+++ b/browser/base/content/browser-addons.js
@@ -525,7 +525,7 @@ var gXPInstallObserver = {
     Services.console.logMessage(consoleMsg);
   },
 
-  async observe(aSubject, aTopic, aData) {
+  async observe(aSubject, aTopic) {
     var installInfo = aSubject.wrappedJSObject;
     var browser = installInfo.browser;
 
@@ -914,9 +914,9 @@ var gXPInstallObserver = {
           let height = undefined;
 
           if (PopupNotifications.isPanelOpen) {
-            let rect = document
-              .getElementById("addon-progress-notification")
-              .getBoundingClientRect();
+            let rect = window.windowUtils.getBoundsWithoutFlushing(
+              document.getElementById("addon-progress-notification")
+            );
             height = rect.height;
           }
 
@@ -1009,7 +1009,7 @@ var gExtensionsNotifications = {
 
     let items = 0;
     if (lazy.AMBrowserExtensionsImport.canCompleteOrCancelInstalls) {
-      this._createAddonButton("webext-imported-addons", null, evt => {
+      this._createAddonButton("webext-imported-addons", null, () => {
         lazy.AMBrowserExtensionsImport.completeInstalls();
       });
       items++;
@@ -1022,7 +1022,7 @@ var gExtensionsNotifications = {
       this._createAddonButton(
         "webext-perms-update-menu-item",
         update.addon,
-        evt => {
+        () => {
           ExtensionsUI.showUpdate(gBrowser, update);
         }
       );
@@ -1032,7 +1032,7 @@ var gExtensionsNotifications = {
       if (++items > 4) {
         break;
       }
-      this._createAddonButton("webext-perms-sideload-menu-item", addon, evt => {
+      this._createAddonButton("webext-perms-sideload-menu-item", addon, () => {
         // We need to hide the main menu manually because the toolbarbutton is
         // removed immediately while processing this event, and PanelUI is
         // unable to identify which panel should be closed automatically.
@@ -1046,16 +1046,11 @@ var gExtensionsNotifications = {
 var BrowserAddonUI = {
   async promptRemoveExtension(addon) {
     let { name } = addon;
-    let [title, btnTitle, message] = await lazy.l10n.formatValues([
+    let [title, btnTitle] = await lazy.l10n.formatValues([
       { id: "addon-removal-title", args: { name } },
       { id: "addon-removal-button" },
-      { id: "addon-removal-message", args: { name } },
     ]);
 
-    if (Services.prefs.getBoolPref("prompts.windowPromptSubDialog", false)) {
-      message = null;
-    }
-
     let {
       BUTTON_TITLE_IS_STRING: titleString,
       BUTTON_TITLE_CANCEL: titleCancel,
@@ -1082,7 +1077,7 @@ var BrowserAddonUI = {
     let result = confirmEx(
       window,
       title,
-      message,
+      null,
       btnFlags,
       btnTitle,
       /* button1 */ null,
@@ -1117,7 +1112,7 @@ var BrowserAddonUI = {
     win.openAbuseReport({ addonId, reportEntryPoint });
   },
 
-  async removeAddon(addonId, eventObject) {
+  async removeAddon(addonId) {
     let addon = addonId && (await AddonManager.getAddonByID(addonId));
     if (!addon || !(addon.permissions & AddonManager.PERM_CAN_UNINSTALL)) {
       return;
@@ -1136,7 +1131,7 @@ var BrowserAddonUI = {
     }
   },
 
-  async manageAddon(addonId, eventObject) {
+  async manageAddon(addonId) {
     let addon = addonId && (await AddonManager.getAddonByID(addonId));
     if (!addon) {
       return;
@@ -1798,7 +1793,7 @@ var gUnifiedExtensions = {
     }
   },
 
-  onWidgetAdded(aWidgetId, aArea, aPosition) {
+  onWidgetAdded(aWidgetId, aArea) {
     // When we pin a widget to the toolbar from a narrow window, the widget
     // will be overflowed directly. In this case, we do not want to change the
     // class name since it is going to be changed by `onWidgetOverflow()`
@@ -1813,7 +1808,7 @@ var gUnifiedExtensions = {
     this._updateWidgetClassName(aWidgetId, inPanel);
   },
 
-  onWidgetOverflow(aNode, aContainer) {
+  onWidgetOverflow(aNode) {
     // We register a CUI listener for each window so we make sure that we
     // handle the event for the right window here.
     if (window !== aNode.ownerGlobal) {
@@ -1823,7 +1818,7 @@ var gUnifiedExtensions = {
     this._updateWidgetClassName(aNode.getAttribute("widget-id"), true);
   },
 
-  onWidgetUnderflow(aNode, aContainer) {
+  onWidgetUnderflow(aNode) {
     // We register a CUI listener for each window so we make sure that we
     // handle the event for the right window here.
     if (window !== aNode.ownerGlobal) {
diff --git a/browser/base/content/browser-allTabsMenu.js b/browser/base/content/browser-allTabsMenu.js
index f11d4da71d..a9caeffca0 100644
--- a/browser/base/content/browser-allTabsMenu.js
+++ b/browser/base/content/browser-allTabsMenu.js
@@ -58,7 +58,7 @@ var gTabsPanel = {
       dropIndicator: this.dropIndicator,
     });
 
-    this.allTabsView.addEventListener("ViewShowing", e => {
+    this.allTabsView.addEventListener("ViewShowing", () => {
       PanelUI._ensureShortcutsShown(this.allTabsView);
 
       let containersEnabled =
@@ -74,7 +74,7 @@ var gTabsPanel = {
         !hasHiddenTabs;
     });
 
-    this.allTabsView.addEventListener("ViewShown", e =>
+    this.allTabsView.addEventListener("ViewShown", () =>
       this.allTabsView
         .querySelector(".all-tabs-item[selected]")
         ?.scrollIntoView({ block: "center" })
@@ -170,7 +170,7 @@ var gTabsPanel = {
     }
     this.allTabsView.addEventListener(
       "ViewShown",
-      e => {
+      () => {
         PanelUI.showSubView(
           this.kElements.hiddenTabsView,
           this.hiddenTabsButton
diff --git a/browser/base/content/browser-box.inc.xhtml b/browser/base/content/browser-box.inc.xhtml
index d445abe7e7..41258d81bb 100644
--- a/browser/base/content/browser-box.inc.xhtml
+++ b/browser/base/content/browser-box.inc.xhtml
@@ -3,6 +3,7 @@
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 <hbox flex="1" id="browser">
+  <html:sidebar-launcher id="sidebar-launcher" flex="1" hidden="true"></html:sidebar-launcher>
   <vbox id="sidebar-box" hidden="true" class="chromeclass-extrachrome">
     <box id="sidebar-header" align="center">
       <toolbarbutton id="sidebar-switcher-target" class="tabbable" aria-expanded="false">
diff --git a/browser/base/content/browser-captivePortal.js b/browser/base/content/browser-captivePortal.js
index 247f8c397f..1fd5497273 100644
--- a/browser/base/content/browser-captivePortal.js
+++ b/browser/base/content/browser-captivePortal.js
@@ -95,7 +95,7 @@ var CaptivePortalWatcher = {
     }
   },
 
-  observe(aSubject, aTopic, aData) {
+  observe(aSubject, aTopic) {
     switch (aTopic) {
       case "captive-portal-login":
         this._captivePortalDetected();
diff --git a/browser/base/content/browser-commands.js b/browser/base/content/browser-commands.js
new file mode 100644
index 0000000000..40eb4d5baa
--- /dev/null
+++ b/browser/base/content/browser-commands.js
@@ -0,0 +1,576 @@
+/* -*- 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/. */
+
+/* eslint-env mozilla/browser-window */
+
+"use strict";
+
+var kSkipCacheFlags =
+  Ci.nsIWebNavigation.LOAD_FLAGS_BYPASS_PROXY |
+  Ci.nsIWebNavigation.LOAD_FLAGS_BYPASS_CACHE;
+
+var BrowserCommands = {
+  back(aEvent) {
+    const where = BrowserUtils.whereToOpenLink(aEvent, false, true);
+
+    if (where == "current") {
+      try {
+        gBrowser.goBack();
+      } catch (ex) {}
+    } else {
+      duplicateTabIn(gBrowser.selectedTab, where, -1);
+    }
+  },
+
+  forward(aEvent) {
+    const where = BrowserUtils.whereToOpenLink(aEvent, false, true);
+
+    if (where == "current") {
+      try {
+        gBrowser.goForward();
+      } catch (ex) {}
+    } else {
+      duplicateTabIn(gBrowser.selectedTab, where, 1);
+    }
+  },
+
+  handleBackspace() {
+    switch (Services.prefs.getIntPref("browser.backspace_action")) {
+      case 0:
+        this.back();
+        break;
+      case 1:
+        goDoCommand("cmd_scrollPageUp");
+        break;
+    }
+  },
+
+  handleShiftBackspace() {
+    switch (Services.prefs.getIntPref("browser.backspace_action")) {
+      case 0:
+        this.forward();
+        break;
+      case 1:
+        goDoCommand("cmd_scrollPageDown");
+        break;
+    }
+  },
+
+  gotoHistoryIndex(aEvent) {
+    aEvent = BrowserUtils.getRootEvent(aEvent);
+
+    const index = aEvent.target.getAttribute("index");
+    if (!index) {
+      return false;
+    }
+
+    const where = BrowserUtils.whereToOpenLink(aEvent);
+
+    if (where == "current") {
+      // Normal click. Go there in the current tab and update session history.
+
+      try {
+        gBrowser.gotoIndex(index);
+      } catch (ex) {
+        return false;
+      }
+      return true;
+    }
+    // Modified click. Go there in a new tab/window.
+
+    const historyindex = aEvent.target.getAttribute("historyindex");
+    duplicateTabIn(gBrowser.selectedTab, where, Number(historyindex));
+    return true;
+  },
+
+  reloadOrDuplicate(aEvent) {
+    aEvent = BrowserUtils.getRootEvent(aEvent);
+    const accelKeyPressed =
+      AppConstants.platform == "macosx" ? aEvent.metaKey : aEvent.ctrlKey;
+    const backgroundTabModifier = aEvent.button == 1 || accelKeyPressed;
+
+    if (aEvent.shiftKey && !backgroundTabModifier) {
+      this.reloadSkipCache();
+      return;
+    }
+
+    const where = BrowserUtils.whereToOpenLink(aEvent, false, true);
+    if (where == "current") {
+      this.reload();
+    } else {
+      duplicateTabIn(gBrowser.selectedTab, where);
+    }
+  },
+
+  reload() {
+    if (gBrowser.currentURI.schemeIs("view-source")) {
+      // Bug 1167797: For view source, we always skip the cache
+      this.reloadSkipCache();
+      return;
+    }
+    this.reloadWithFlags(Ci.nsIWebNavigation.LOAD_FLAGS_NONE);
+  },
+
+  reloadSkipCache() {
+    // Bypass proxy and cache.
+    this.reloadWithFlags(kSkipCacheFlags);
+  },
+
+  reloadWithFlags(reloadFlags) {
+    const unchangedRemoteness = [];
+
+    for (const tab of gBrowser.selectedTabs) {
+      const browser = tab.linkedBrowser;
+      const url = browser.currentURI;
+      const urlSpec = url.spec;
+      // We need to cache the content principal here because the browser will be
+      // reconstructed when the remoteness changes and the content prinicpal will
+      // be cleared after reconstruction.
+      const principal = tab.linkedBrowser.contentPrincipal;
+      if (gBrowser.updateBrowserRemotenessByURL(browser, urlSpec)) {
+        // If the remoteness has changed, the new browser doesn't have any
+        // information of what was loaded before, so we need to load the previous
+        // URL again.
+        if (tab.linkedPanel) {
+          loadBrowserURI(browser, url, principal);
+        } else {
+          // Shift to fully loaded browser and make
+          // sure load handler is instantiated.
+          tab.addEventListener(
+            "SSTabRestoring",
+            () => loadBrowserURI(browser, url, principal),
+            { once: true }
+          );
+          gBrowser._insertBrowser(tab);
+        }
+      } else {
+        unchangedRemoteness.push(tab);
+      }
+    }
+
+    if (!unchangedRemoteness.length) {
+      return;
+    }
+
+    // Reset temporary permissions on the remaining tabs to reload.
+    // This is done here because we only want to reset
+    // permissions on user reload.
+    for (const tab of unchangedRemoteness) {
+      SitePermissions.clearTemporaryBlockPermissions(tab.linkedBrowser);
+      // Also reset DOS mitigations for the basic auth prompt on reload.
+      delete tab.linkedBrowser.authPromptAbuseCounter;
+    }
+    gIdentityHandler.hidePopup();
+    gPermissionPanel.hidePopup();
+
+    const handlingUserInput = document.hasValidTransientUserGestureActivation;
+
+    for (const tab of unchangedRemoteness) {
+      if (tab.linkedPanel) {
+        sendReloadMessage(tab);
+      } else {
+        // Shift to fully loaded browser and make
+        // sure load handler is instantiated.
+        tab.addEventListener("SSTabRestoring", () => sendReloadMessage(tab), {
+          once: true,
+        });
+        gBrowser._insertBrowser(tab);
+      }
+    }
+
+    function loadBrowserURI(browser, url, principal) {
+      browser.loadURI(url, {
+        flags: reloadFlags,
+        triggeringPrincipal: principal,
+      });
+    }
+
+    function sendReloadMessage(tab) {
+      tab.linkedBrowser.sendMessageToActor(
+        "Browser:Reload",
+        { flags: reloadFlags, handlingUserInput },
+        "BrowserTab"
+      );
+    }
+  },
+
+  stop() {
+    gBrowser.webNavigation.stop(Ci.nsIWebNavigation.STOP_ALL);
+  },
+
+  home(aEvent) {
+    if (aEvent?.button == 2) {
+      // right-click: do nothing
+      return;
+    }
+
+    const homePage = HomePage.get(window);
+    let where = BrowserUtils.whereToOpenLink(aEvent, false, true);
+
+    // Don't load the home page in pinned or hidden tabs (e.g. Firefox View).
+    if (
+      where == "current" &&
+      (gBrowser?.selectedTab.pinned || gBrowser?.selectedTab.hidden)
+    ) {
+      where = "tab";
+    }
+
+    // openTrustedLinkIn in utilityOverlay.js doesn't handle loading multiple pages
+    let notifyObservers;
+    switch (where) {
+      case "current":
+        // If we're going to load an initial page in the current tab as the
+        // home page, we set initialPageLoadedFromURLBar so that the URL
+        // bar is cleared properly (even during a remoteness flip).
+        if (isInitialPage(homePage)) {
+          gBrowser.selectedBrowser.initialPageLoadedFromUserAction = homePage;
+        }
+        loadOneOrMoreURIs(
+          homePage,
+          Services.scriptSecurityManager.getSystemPrincipal(),
+          null
+        );
+        if (isBlankPageURL(homePage)) {
+          gURLBar.select();
+        } else {
+          gBrowser.selectedBrowser.focus();
+        }
+        notifyObservers = true;
+        aEvent?.preventDefault();
+        break;
+      case "tabshifted":
+      case "tab": {
+        const urls = homePage.split("|");
+        const loadInBackground = Services.prefs.getBoolPref(
+          "browser.tabs.loadBookmarksInBackground",
+          false
+        );
+        // The homepage observer event should only be triggered when the homepage opens
+        // in the foreground. This is mostly to support the homepage changed by extension
+        // doorhanger which doesn't currently support background pages. This may change in
+        // bug 1438396.
+        notifyObservers = !loadInBackground;
+        gBrowser.loadTabs(urls, {
+          inBackground: loadInBackground,
+          triggeringPrincipal:
+            Services.scriptSecurityManager.getSystemPrincipal(),
+          csp: null,
+        });
+        if (!loadInBackground) {
+          if (isBlankPageURL(homePage)) {
+            gURLBar.select();
+          } else {
+            gBrowser.selectedBrowser.focus();
+          }
+        }
+        aEvent?.preventDefault();
+        break;
+      }
+      case "window":
+        // OpenBrowserWindow will trigger the observer event, so no need to do so here.
+        notifyObservers = false;
+        OpenBrowserWindow();
+        aEvent?.preventDefault();
+        break;
+    }
+
+    if (notifyObservers) {
+      // A notification for when a user has triggered their homepage. This is used
+      // to display a doorhanger explaining that an extension has modified the
+      // homepage, if necessary. Observers are only notified if the homepage
+      // becomes the active page.
+      Services.obs.notifyObservers(null, "browser-open-homepage-start");
+    }
+  },
+
+  openTab({ event, url } = {}) {
+    let werePassedURL = !!url;
+    url ??= BROWSER_NEW_TAB_URL;
+    let searchClipboard =
+      gMiddleClickNewTabUsesPasteboard && event?.button == 1;
+
+    let relatedToCurrent = false;
+    let where = "tab";
+
+    if (event) {
+      where = whereToOpenLink(event, false, true);
+
+      switch (where) {
+        case "tab":
+        case "tabshifted":
+          // When accel-click or middle-click are used, open the new tab as
+          // related to the current tab.
+          relatedToCurrent = true;
+          break;
+        case "current":
+          where = "tab";
+          break;
+      }
+    }
+
+    // A notification intended to be useful for modular peformance tracking
+    // starting as close as is reasonably possible to the time when the user
+    // expressed the intent to open a new tab.  Since there are a lot of
+    // entry points, this won't catch every single tab created, but most
+    // initiated by the user should go through here.
+    //
+    // Note 1: This notification gets notified with a promise that resolves
+    //         with the linked browser when the tab gets created
+    // Note 2: This is also used to notify a user that an extension has changed
+    //         the New Tab page.
+    Services.obs.notifyObservers(
+      {
+        wrappedJSObject: new Promise(resolve => {
+          let options = {
+            relatedToCurrent,
+            resolveOnNewTabCreated: resolve,
+          };
+          if (!werePassedURL && searchClipboard) {
+            let clipboard = readFromClipboard();
+            clipboard =
+              UrlbarUtils.stripUnsafeProtocolOnPaste(clipboard).trim();
+            if (clipboard) {
+              url = clipboard;
+              options.allowThirdPartyFixup = true;
+            }
+          }
+          openTrustedLinkIn(url, where, options);
+        }),
+      },
+      "browser-open-newtab-start"
+    );
+  },
+
+  openFileWindow() {
+    // Get filepicker component.
+    try {
+      const nsIFilePicker = Ci.nsIFilePicker;
+      const fp = Cc["@mozilla.org/filepicker;1"].createInstance(nsIFilePicker);
+      const fpCallback = function fpCallback_done(aResult) {
+        if (aResult == nsIFilePicker.returnOK) {
+          try {
+            if (fp.file) {
+              gLastOpenDirectory.path = fp.file.parent.QueryInterface(
+                Ci.nsIFile
+              );
+            }
+          } catch (ex) {}
+          openTrustedLinkIn(fp.fileURL.spec, "current");
+        }
+      };
+
+      fp.init(
+        window.browsingContext,
+        gNavigatorBundle.getString("openFile"),
+        nsIFilePicker.modeOpen
+      );
+      fp.appendFilters(
+        nsIFilePicker.filterAll |
+          nsIFilePicker.filterText |
+          nsIFilePicker.filterImages |
+          nsIFilePicker.filterXML |
+          nsIFilePicker.filterHTML |
+          nsIFilePicker.filterPDF
+      );
+      fp.displayDirectory = gLastOpenDirectory.path;
+      fp.open(fpCallback);
+    } catch (ex) {}
+  },
+
+  closeTabOrWindow(event) {
+    // If we're not a browser window, just close the window.
+    if (window.location.href != AppConstants.BROWSER_CHROME_URL) {
+      closeWindow(true);
+      return;
+    }
+
+    // In a multi-select context, close all selected tabs
+    if (gBrowser.multiSelectedTabsCount) {
+      gBrowser.removeMultiSelectedTabs();
+      return;
+    }
+
+    // Keyboard shortcuts that would close a tab that is pinned select the first
+    // unpinned tab instead.
+    if (
+      event &&
+      (event.ctrlKey || event.metaKey || event.altKey) &&
+      gBrowser.selectedTab.pinned
+    ) {
+      if (gBrowser.visibleTabs.length > gBrowser._numPinnedTabs) {
+        gBrowser.tabContainer.selectedIndex = gBrowser._numPinnedTabs;
+      }
+      return;
+    }
+
+    // If the current tab is the last one, this will close the window.
+    gBrowser.removeCurrentTab({ animate: true });
+  },
+
+  tryToCloseWindow(event) {
+    if (WindowIsClosing(event)) {
+      window.close();
+    } // WindowIsClosing does all the necessary checks
+  },
+
+  /**
+   * Open the View Source dialog.
+   *
+   * @param args
+   *        An object with the following properties:
+   *
+   *        URL (required):
+   *          A string URL for the page we'd like to view the source of.
+   *        browser (optional):
+   *          The browser containing the document that we would like to view the
+   *          source of. This is required if outerWindowID is passed.
+   *        outerWindowID (optional):
+   *          The outerWindowID of the content window containing the document that
+   *          we want to view the source of. You only need to provide this if you
+   *          want to attempt to retrieve the document source from the network
+   *          cache.
+   *        lineNumber (optional):
+   *          The line number to focus on once the source is loaded.
+   */
+  async viewSourceOfDocument(args) {
+    // Check if external view source is enabled.  If so, try it.  If it fails,
+    // fallback to internal view source.
+    if (Services.prefs.getBoolPref("view_source.editor.external")) {
+      try {
+        await top.gViewSourceUtils.openInExternalEditor(args);
+        return;
+      } catch (data) {}
+    }
+
+    let tabBrowser = gBrowser;
+    let preferredRemoteType;
+    let initialBrowsingContextGroupId;
+    if (args.browser) {
+      preferredRemoteType = args.browser.remoteType;
+      initialBrowsingContextGroupId = args.browser.browsingContext.group.id;
+    } else {
+      if (!tabBrowser) {
+        throw new Error(
+          "viewSourceOfDocument should be passed the " +
+            "subject browser if called from a window without " +
+            "gBrowser defined."
+        );
+      }
+      // Some internal URLs (such as specific chrome: and about: URLs that are
+      // not yet remote ready) cannot be loaded in a remote browser.  View
+      // source in tab expects the new view source browser's remoteness to match
+      // that of the original URL, so disable remoteness if necessary for this
+      // URL.
+      const oa = E10SUtils.predictOriginAttributes({ window });
+      preferredRemoteType = E10SUtils.getRemoteTypeForURI(
+        args.URL,
+        gMultiProcessBrowser,
+        gFissionBrowser,
+        E10SUtils.DEFAULT_REMOTE_TYPE,
+        null,
+        oa
+      );
+    }
+
+    // In the case of popups, we need to find a non-popup browser window.
+    if (!tabBrowser || !window.toolbar.visible) {
+      // This returns only non-popup browser windows by default.
+      const browserWindow = BrowserWindowTracker.getTopWindow();
+      tabBrowser = browserWindow.gBrowser;
+    }
+
+    const inNewWindow = !Services.prefs.getBoolPref("view_source.tab");
+
+    // `viewSourceInBrowser` will load the source content from the page
+    // descriptor for the tab (when possible) or fallback to the network if
+    // that fails.  Either way, the view source module will manage the tab's
+    // location, so use "about:blank" here to avoid unnecessary redundant
+    // requests.
+    const tab = tabBrowser.addTab("about:blank", {
+      relatedToCurrent: true,
+      inBackground: inNewWindow,
+      skipAnimation: inNewWindow,
+      preferredRemoteType,
+      initialBrowsingContextGroupId,
+      triggeringPrincipal: Services.scriptSecurityManager.getSystemPrincipal(),
+      skipLoad: true,
+    });
+    args.viewSourceBrowser = tabBrowser.getBrowserForTab(tab);
+    top.gViewSourceUtils.viewSourceInBrowser(args);
+
+    if (inNewWindow) {
+      tabBrowser.hideTab(tab);
+      tabBrowser.replaceTabWithWindow(tab);
+    }
+  },
+
+  /**
+   * Opens the View Source dialog for the source loaded in the root
+   * top-level document of the browser. This is really just a
+   * convenience wrapper around viewSourceOfDocument.
+   *
+   * @param browser
+   *        The browser that we want to load the source of.
+   */
+  viewSource(browser) {
+    this.viewSourceOfDocument({
+      browser,
+      outerWindowID: browser.outerWindowID,
+      URL: browser.currentURI.spec,
+    });
+  },
+
+  /**
+   * @param documentURL URL of the document to view, or null for this window's document
+   * @param initialTab name of the initial tab to display, or null for the first tab
+   * @param imageElement image to load in the Media Tab of the Page Info window; can be null/omitted
+   * @param browsingContext the browsingContext of the frame that we want to view information about; can be null/omitted
+   * @param browser the browser containing the document we're interested in inspecting; can be null/omitted
+   */
+  pageInfo(documentURL, initialTab, imageElement, browsingContext, browser) {
+    const args = { initialTab, imageElement, browsingContext, browser };
+
+    documentURL =
+      documentURL || window.gBrowser.selectedBrowser.currentURI.spec;
+
+    const isPrivate = PrivateBrowsingUtils.isWindowPrivate(window);
+
+    // Check for windows matching the url
+    for (const currentWindow of Services.wm.getEnumerator(
+      "Browser:page-info"
+    )) {
+      if (currentWindow.closed) {
+        continue;
+      }
+      if (
+        currentWindow.document.documentElement.getAttribute("relatedUrl") ==
+          documentURL &&
+        PrivateBrowsingUtils.isWindowPrivate(currentWindow) == isPrivate
+      ) {
+        currentWindow.focus();
+        currentWindow.resetPageInfo(args);
+        return currentWindow;
+      }
+    }
+
+    // We didn't find a matching window, so open a new one.
+    let options = "chrome,toolbar,dialog=no,resizable";
+
+    // Ensure the window groups correctly in the Windows taskbar
+    if (isPrivate) {
+      options += ",private";
+    }
+    return openDialog(
+      "chrome://browser/content/pageinfo/pageInfo.xhtml",
+      "",
+      options,
+      args
+    );
+  },
+
+  fullScreen() {
+    window.fullScreen = !window.fullScreen || BrowserHandler.kiosk;
+  },
+};
diff --git a/browser/base/content/browser-context.inc b/browser/base/content/browser-context.inc
index ff4015e3d4..3ce284a52f 100644
--- a/browser/base/content/browser-context.inc
+++ b/browser/base/content/browser-context.inc
@@ -439,7 +439,7 @@
                 oncommand="gContextMenu.viewPartialSource();"/>
       <menuitem id="context-viewsource"
                 data-l10n-id="main-context-menu-view-page-source"
-                oncommand="BrowserViewSource(gContextMenu.browser);"/>
+                oncommand="BrowserCommands.viewSource(gContextMenu.browser);"/>
       <menuitem id="context-inspect-a11y"
                 hidden="true"
                 data-l10n-id="main-context-menu-inspect-a11y-properties"
diff --git a/browser/base/content/browser-ctrlTab.js b/browser/base/content/browser-ctrlTab.js
index d4d79a6886..e5d16e605b 100644
--- a/browser/base/content/browser-ctrlTab.js
+++ b/browser/base/content/browser-ctrlTab.js
@@ -284,7 +284,7 @@ var ctrlTab = {
       this.uninit();
     }
   },
-  observe(aSubject, aTopic, aPrefName) {
+  observe() {
     this.readPref();
   },
 
@@ -654,7 +654,7 @@ var ctrlTab = {
         // tab attribute modified (i.e. label, busy, image)
         // update preview only if tab attribute modified in the list
         if (
-          event.detail.changed.some((elem, ind, arr) =>
+          event.detail.changed.some(elem =>
             ["label", "busy", "image"].includes(elem)
           )
         ) {
diff --git a/browser/base/content/browser-data-submission-info-bar.js b/browser/base/content/browser-data-submission-info-bar.js
index 26d9affb29..104414d582 100644
--- a/browser/base/content/browser-data-submission-info-bar.js
+++ b/browser/base/content/browser-data-submission-info-bar.js
@@ -92,7 +92,7 @@ var gDataNotificationInfoBar = {
     }
   },
 
-  observe(subject, topic, data) {
+  observe(subject, topic) {
     switch (topic) {
       case "datareporting:notify-data-policy:request":
         let request = subject.wrappedJSObject.object;
diff --git a/browser/base/content/browser-fullScreenAndPointerLock.js b/browser/base/content/browser-fullScreenAndPointerLock.js
index aae596a0f7..c8794c760c 100644
--- a/browser/base/content/browser-fullScreenAndPointerLock.js
+++ b/browser/base/content/browser-fullScreenAndPointerLock.js
@@ -365,26 +365,19 @@ var FullScreen = {
       passive: true,
     });
 
-    if (enterFS) {
-      gNavToolbox.setAttribute("inFullscreen", true);
-      document.documentElement.setAttribute("inFullscreen", true);
-      let alwaysUsesNativeFullscreen =
+    document.documentElement.toggleAttribute("inFullscreen", enterFS);
+    document.documentElement.toggleAttribute(
+      "macOSNativeFullscreen",
+      enterFS &&
         AppConstants.platform == "macosx" &&
-        Services.prefs.getBoolPref("full-screen-api.macos-native-full-screen");
-      if (
-        (alwaysUsesNativeFullscreen || !document.fullscreenElement) &&
-        AppConstants.platform == "macosx"
-      ) {
-        document.documentElement.setAttribute("macOSNativeFullscreen", true);
-      }
-    } else {
-      gNavToolbox.removeAttribute("inFullscreen");
-      document.documentElement.removeAttribute("inFullscreen");
-      document.documentElement.removeAttribute("macOSNativeFullscreen");
-    }
+        (Services.prefs.getBoolPref(
+          "full-screen-api.macos-native-full-screen"
+        ) ||
+          !document.fullscreenElement)
+    );
 
     if (!document.fullscreenElement) {
-      this._updateToolbars(enterFS);
+      ToolbarIconColor.inferFromText("fullscreen", enterFS);
     }
 
     if (enterFS) {
@@ -948,22 +941,6 @@ var FullScreen = {
 
     MousePosTracker.removeListener(this);
   },
-
-  _updateToolbars(aEnterFS) {
-    for (let el of document.querySelectorAll(
-      "toolbar[fullscreentoolbar=true]"
-    )) {
-      // Set the inFullscreen attribute to allow specific styling
-      // in fullscreen mode
-      if (aEnterFS) {
-        el.setAttribute("inFullscreen", true);
-      } else {
-        el.removeAttribute("inFullscreen");
-      }
-    }
-
-    ToolbarIconColor.inferFromText("fullscreen", aEnterFS);
-  },
 };
 
 ChromeUtils.defineLazyGetter(FullScreen, "_permissionNotificationIDs", () => {
diff --git a/browser/base/content/browser-gestureSupport.js b/browser/base/content/browser-gestureSupport.js
index 0ff42f5160..6bec059789 100644
--- a/browser/base/content/browser-gestureSupport.js
+++ b/browser/base/content/browser-gestureSupport.js
@@ -269,7 +269,7 @@ var gGestureSupport = {
       gHistorySwipeAnimation.updateAnimation(aEvent.delta);
     };
 
-    this._doEnd = function GS__doEnd(aEvent) {
+    this._doEnd = function GS__doEnd() {
       gHistorySwipeAnimation.swipeEndEventReceived();
 
       this._doUpdate = function () {};
@@ -393,7 +393,7 @@ var gGestureSupport = {
    * @param aEvent
    *        The continual motion update event to handle
    */
-  _doUpdate(aEvent) {},
+  _doUpdate() {},
 
   /**
    * Handle gesture end events.  This function will be set by _setupSwipe.
@@ -401,7 +401,7 @@ var gGestureSupport = {
    * @param aEvent
    *        The gesture end event to handle
    */
-  _doEnd(aEvent) {},
+  _doEnd() {},
 
   /**
    * Convert the swipe gesture into a browser action based on the direction.
@@ -874,7 +874,7 @@ var gHistorySwipeAnimation = {
     }
   },
 
-  _completeFadeOut: function HSA__completeFadeOut(aEvent) {
+  _completeFadeOut: function HSA__completeFadeOut() {
     if (!this._isStoppingAnimation) {
       // The animation was restarted in the middle of our stopping fade out
       // tranistion, so don't do anything.
@@ -943,7 +943,7 @@ var gHistorySwipeAnimation = {
     return element;
   },
 
-  observe(subj, topic, data) {
+  observe(subj, topic) {
     switch (topic) {
       case "nsPref:changed":
         this._initPrefValues();
diff --git a/browser/base/content/browser-menubar.inc b/browser/base/content/browser-menubar.inc
index 66dd662137..0c8b4a1cf6 100644
--- a/browser/base/content/browser-menubar.inc
+++ b/browser/base/content/browser-menubar.inc
@@ -140,7 +140,7 @@
                   </menupopup>
                 </menu>
                 <menu id="viewSidebarMenuMenu" data-l10n-id="menu-view-sidebar">
-                  <menupopup id="viewSidebarMenu">
+                  <menupopup id="viewSidebarMenu" onpopupshowing="SidebarUI.setMegalistMenubarVisibility(event);">
                     <menuitem id="menu_bookmarksSidebar"
                               type="checkbox"
                               key="viewBookmarksSidebarKb"
@@ -153,6 +153,9 @@
                               type="checkbox"
                               class="sync-ui-item"
                               oncommand="SidebarUI.toggle('viewTabsSidebar');" data-l10n-id="menu-view-synced-tabs-sidebar"/>
+                    <menuitem id="menu_megalistSidebar"
+                              type="checkbox"
+                              oncommand="SidebarUI.toggle('viewMegalistSidebar');" data-l10n-id="menu-view-megalist-sidebar"/>
                   </menupopup>
                 </menu>
                 <menuseparator/>
diff --git a/browser/base/content/browser-pageActions.js b/browser/base/content/browser-pageActions.js
index 1cc895434d..1fd1062948 100644
--- a/browser/base/content/browser-pageActions.js
+++ b/browser/base/content/browser-pageActions.js
@@ -1008,7 +1008,7 @@ BrowserPageActions.bookmark = {
     }
   },
 
-  onCommand(event, buttonNode) {
+  onCommand(event) {
     PanelMultiView.hidePopup(BrowserPageActions.panelNode);
     BookmarkingUI.onStarCommand(event);
   },
diff --git a/browser/base/content/browser-places.js b/browser/base/content/browser-places.js
index 58e61f7bf7..c940fade5f 100644
--- a/browser/base/content/browser-places.js
+++ b/browser/base/content/browser-places.js
@@ -17,7 +17,7 @@ XPCOMUtils.defineLazyPreferenceGetter(
   "SHOW_OTHER_BOOKMARKS",
   "browser.toolbars.bookmarks.showOtherBookmarks",
   true,
-  (aPref, aPrevVal, aNewVal) => {
+  () => {
     BookmarkingUI.maybeShowOtherBookmarksFolder().then(() => {
       document
         .getElementById("PlacesToolbar")
@@ -253,7 +253,7 @@ var StarUI = {
       }
       target.addEventListener(
         "popupshown",
-        function (event) {
+        function () {
           fn();
         },
         { capture: true, once: true }
@@ -1174,7 +1174,7 @@ var PlacesToolbarHelper = {
     return null;
   },
 
-  onWidgetUnderflow(aNode, aContainer) {
+  onWidgetUnderflow(aNode) {
     // The view gets broken by being removed and reinserted by the overflowable
     // toolbar, so we have to force an uninit and reinit.
     let win = aNode.ownerGlobal;
@@ -1183,7 +1183,7 @@ var PlacesToolbarHelper = {
     }
   },
 
-  onWidgetAdded(aWidgetId, aArea, aPosition) {
+  onWidgetAdded(aWidgetId) {
     if (aWidgetId == "personal-bookmarks" && !this._isCustomizing) {
       // It's possible (with the "Add to Menu", "Add to Toolbar" context
       // options) that the Places Toolbar Items have been moved without
@@ -1659,13 +1659,13 @@ var BookmarkingUI = {
     }
   },
 
-  onWidgetReset: function BUI_widgetReset(aNode, aContainer) {
+  onWidgetReset: function BUI_widgetReset(aNode) {
     if (aNode == this.button) {
       this._onWidgetWasMoved();
     }
   },
 
-  onWidgetUndoMove: function BUI_undoWidgetUndoMove(aNode, aContainer) {
+  onWidgetUndoMove: function BUI_undoWidgetUndoMove(aNode) {
     if (aNode == this.button) {
       this._onWidgetWasMoved();
     }
@@ -2155,7 +2155,7 @@ var BookmarkingUI = {
     });
   },
 
-  onWidgetUnderflow(aNode, aContainer) {
+  onWidgetUnderflow(aNode) {
     let win = aNode.ownerGlobal;
     if (aNode.id != this.BOOKMARK_BUTTON_ID || win != window) {
       return;
diff --git a/browser/base/content/browser-sets.inc b/browser/base/content/browser-sets.inc
index 090e94b684..e247f0641e 100644
--- a/browser/base/content/browser-sets.inc
+++ b/browser/base/content/browser-sets.inc
@@ -16,12 +16,12 @@
 
   <commandset id="mainCommandSet">
     <command id="cmd_newNavigator" oncommand="OpenBrowserWindow()"/>
-    <command id="cmd_handleBackspace" oncommand="BrowserHandleBackspace();" />
-    <command id="cmd_handleShiftBackspace" oncommand="BrowserHandleShiftBackspace();" />
+    <command id="cmd_handleBackspace" oncommand="BrowserCommands.handleBackspace();" />
+    <command id="cmd_handleShiftBackspace" oncommand="BrowserCommands.handleShiftBackspace();" />
 
-    <command id="cmd_newNavigatorTab" oncommand="BrowserOpenTab({ event });"/>
-    <command id="cmd_newNavigatorTabNoEvent" oncommand="BrowserOpenTab();"/>
-    <command id="Browser:OpenFile"  oncommand="BrowserOpenFileWindow();"/>
+    <command id="cmd_newNavigatorTab" oncommand="BrowserCommands.openTab({ event });"/>
+    <command id="cmd_newNavigatorTabNoEvent" oncommand="BrowserCommands.openTab();"/>
+    <command id="Browser:OpenFile"  oncommand="BrowserCommands.openFileWindow();"/>
     <command id="Browser:SavePage" oncommand="saveBrowser(gBrowser.selectedBrowser);"/>
 
     <command id="Browser:SendLink"
@@ -32,17 +32,17 @@
     <command id="cmd_printPreviewToggle" oncommand="PrintUtils.togglePrintPreview(gBrowser.selectedBrowser.browsingContext);"/>
     <command id="cmd_file_importFromAnotherBrowser" oncommand="MigrationUtils.showMigrationWizard(window, { entrypoint: MigrationUtils.MIGRATION_ENTRYPOINTS.FILE_MENU });"/>
     <command id="cmd_help_importFromAnotherBrowser" oncommand="MigrationUtils.showMigrationWizard(window, { entrypoint: MigrationUtils.MIGRATION_ENTRYPOINTS.HELP_MENU });"/>
-    <command id="cmd_close" oncommand="BrowserCloseTabOrWindow(event);"/>
-    <command id="cmd_closeWindow" oncommand="BrowserTryToCloseWindow(event)"/>
+    <command id="cmd_close" oncommand="BrowserCommands.closeTabOrWindow(event);"/>
+    <command id="cmd_closeWindow" oncommand="BrowserCommands.tryToCloseWindow(event)"/>
     <command id="cmd_toggleMute" oncommand="gBrowser.toggleMuteAudioOnMultiSelectedTabs(gBrowser.selectedTab)"/>
     <command id="cmd_CustomizeToolbars" oncommand="gCustomizeMode.enter()"/>
     <command id="cmd_toggleOfflineStatus" oncommand="BrowserOffline.toggleOfflineStatus();"/>
     <command id="cmd_quitApplication" oncommand="goQuitApplication(event)"/>
 
     <command id="View:AboutProcesses" oncommand="switchToTabHavingURI('about:processes', true)"/>
-    <command id="View:PageSource" oncommand="BrowserViewSource(window.gBrowser.selectedBrowser);"/>
-    <command id="View:PageInfo" oncommand="BrowserPageInfo();"/>
-    <command id="View:FullScreen" oncommand="BrowserFullScreen();"/>
+    <command id="View:PageSource" oncommand="BrowserCommands.viewSource(window.gBrowser.selectedBrowser);"/>
+    <command id="View:PageInfo" oncommand="BrowserCommands.pageInfo();"/>
+    <command id="View:FullScreen" oncommand="BrowserCommands.fullScreen();"/>
     <command id="View:ReaderView" oncommand="AboutReaderParent.toggleReaderMode(event);"/>
     <command id="View:PictureInPicture" oncommand="PictureInPicture.onCommand(event);"/>
     <command id="cmd_find" oncommand="gLazyFindCommand('onFindCommand')"/>
@@ -60,20 +60,20 @@
             oncommand="PlacesCommandHook.searchBookmarks();"/>
     <command id="Browser:BookmarkAllTabs"
              oncommand="PlacesCommandHook.bookmarkTabs();"/>
-    <command id="Browser:Back"    oncommand="BrowserBack();" disabled="true"/>
-    <command id="Browser:BackOrBackDuplicate" oncommand="BrowserBack(event);" disabled="true">
+    <command id="Browser:Back"    oncommand="BrowserCommands.back();" disabled="true"/>
+    <command id="Browser:BackOrBackDuplicate" oncommand="BrowserCommands.back(event);" disabled="true">
       <observes element="Browser:Back" attribute="disabled"/>
     </command>
-    <command id="Browser:Forward" oncommand="BrowserForward();" disabled="true"/>
-    <command id="Browser:ForwardOrForwardDuplicate" oncommand="BrowserForward(event);" disabled="true">
+    <command id="Browser:Forward" oncommand="BrowserCommands.forward();" disabled="true"/>
+    <command id="Browser:ForwardOrForwardDuplicate" oncommand="BrowserCommands.forward(event);" disabled="true">
       <observes element="Browser:Forward" attribute="disabled"/>
     </command>
-    <command id="Browser:Stop"    oncommand="BrowserStop();" disabled="true"/>
-    <command id="Browser:Reload"  oncommand="if (event.shiftKey) BrowserReloadSkipCache(); else BrowserReload()" disabled="true"/>
-    <command id="Browser:ReloadOrDuplicate" oncommand="BrowserReloadOrDuplicate(event)" disabled="true">
+    <command id="Browser:Stop"    oncommand="BrowserCommands.stop();" disabled="true"/>
+    <command id="Browser:Reload"  oncommand="if (event.shiftKey) BrowserCommands.reloadSkipCache(); else BrowserCommands.reload()" disabled="true"/>
+    <command id="Browser:ReloadOrDuplicate" oncommand="BrowserCommands.reloadOrDuplicate(event)" disabled="true">
       <observes element="Browser:Reload" attribute="disabled"/>
     </command>
-    <command id="Browser:ReloadSkipCache" oncommand="BrowserReloadSkipCache()" disabled="true">
+    <command id="Browser:ReloadSkipCache" oncommand="BrowserCommands.reloadSkipCache()" disabled="true">
       <observes element="Browser:Reload" attribute="disabled"/>
     </command>
     <command id="Browser:NextTab" oncommand="gBrowser.tabContainer.advanceSelectedTab(1, true);"/>
@@ -216,7 +216,7 @@
     <key id="goBackKb2" data-l10n-id="nav-back-shortcut-alt" command="Browser:Back" modifiers="accel"/>
     <key id="goForwardKb2" data-l10n-id="nav-fwd-shortcut-alt" command="Browser:Forward" modifiers="accel"/>
 #endif
-    <key id="goHome" keycode="VK_HOME" oncommand="BrowserHome();" modifiers="alt"/>
+    <key id="goHome" keycode="VK_HOME" oncommand="BrowserCommands.home();" modifiers="alt"/>
     <key keycode="VK_F5" command="Browser:Reload"/>
 #ifndef XP_MACOSX
     <key id="showAllHistoryKb" data-l10n-id="history-show-all-shortcut" command="Browser:ShowAllHistory" modifiers="accel,shift"/>
diff --git a/browser/base/content/browser-sidebar.js b/browser/base/content/browser-sidebar.js
deleted file mode 100644
index 2d730700a6..0000000000
--- a/browser/base/content/browser-sidebar.js
+++ /dev/null
@@ -1,674 +0,0 @@
-/* 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/. */
-
-/**
- * SidebarUI controls showing and hiding the browser sidebar.
- */
-var SidebarUI = {
-  get sidebars() {
-    if (this._sidebars) {
-      return this._sidebars;
-    }
-
-    function makeSidebar({ elementId, ...rest }) {
-      return {
-        get sourceL10nEl() {
-          return document.getElementById(elementId);
-        },
-        get title() {
-          return document.getElementById(elementId).getAttribute("label");
-        },
-        ...rest,
-      };
-    }
-
-    return (this._sidebars = new Map([
-      [
-        "viewBookmarksSidebar",
-        makeSidebar({
-          elementId: "sidebar-switcher-bookmarks",
-          url: "chrome://browser/content/places/bookmarksSidebar.xhtml",
-          menuId: "menu_bookmarksSidebar",
-        }),
-      ],
-      [
-        "viewHistorySidebar",
-        makeSidebar({
-          elementId: "sidebar-switcher-history",
-          url: "chrome://browser/content/places/historySidebar.xhtml",
-          menuId: "menu_historySidebar",
-          triggerButtonId: "appMenuViewHistorySidebar",
-        }),
-      ],
-      [
-        "viewTabsSidebar",
-        makeSidebar({
-          elementId: "sidebar-switcher-tabs",
-          url: "chrome://browser/content/syncedtabs/sidebar.xhtml",
-          menuId: "menu_tabsSidebar",
-        }),
-      ],
-    ]));
-  },
-
-  // Avoid getting the browser element from init() to avoid triggering the
-  // <browser> constructor during startup if the sidebar is hidden.
-  get browser() {
-    if (this._browser) {
-      return this._browser;
-    }
-    return (this._browser = document.getElementById("sidebar"));
-  },
-  POSITION_START_PREF: "sidebar.position_start",
-  DEFAULT_SIDEBAR_ID: "viewBookmarksSidebar",
-
-  // lastOpenedId is set in show() but unlike currentID it's not cleared out on hide
-  // and isn't persisted across windows
-  lastOpenedId: null,
-
-  _box: null,
-  // The constructor of this label accesses the browser element due to the
-  // control="sidebar" attribute, so avoid getting this label during startup.
-  get _title() {
-    if (this.__title) {
-      return this.__title;
-    }
-    return (this.__title = document.getElementById("sidebar-title"));
-  },
-  _splitter: null,
-  _reversePositionButton: null,
-  _switcherPanel: null,
-  _switcherTarget: null,
-  _switcherArrow: null,
-  _inited: false,
-
-  /**
-   * @type {MutationObserver | null}
-   */
-  _observer: null,
-
-  _initDeferred: Promise.withResolvers(),
-
-  get promiseInitialized() {
-    return this._initDeferred.promise;
-  },
-
-  get initialized() {
-    return this._inited;
-  },
-
-  init() {
-    this._box = document.getElementById("sidebar-box");
-    this._splitter = document.getElementById("sidebar-splitter");
-    this._reversePositionButton = document.getElementById(
-      "sidebar-reverse-position"
-    );
-    this._switcherPanel = document.getElementById("sidebarMenu-popup");
-    this._switcherTarget = document.getElementById("sidebar-switcher-target");
-    this._switcherArrow = document.getElementById("sidebar-switcher-arrow");
-
-    this._switcherTarget.addEventListener("command", () => {
-      this.toggleSwitcherPanel();
-    });
-    this._switcherTarget.addEventListener("keydown", event => {
-      this.handleKeydown(event);
-    });
-
-    this._inited = true;
-
-    Services.obs.addObserver(this, "intl:app-locales-changed");
-
-    this._initDeferred.resolve();
-  },
-
-  uninit() {
-    // If this is the last browser window, persist various values that should be
-    // remembered for after a restart / reopening a browser window.
-    let enumerator = Services.wm.getEnumerator("navigator:browser");
-    if (!enumerator.hasMoreElements()) {
-      let xulStore = Services.xulStore;
-      xulStore.persist(this._box, "sidebarcommand");
-
-      if (this._box.hasAttribute("positionend")) {
-        xulStore.persist(this._box, "positionend");
-      } else {
-        xulStore.removeValue(
-          document.documentURI,
-          "sidebar-box",
-          "positionend"
-        );
-      }
-      if (this._box.hasAttribute("checked")) {
-        xulStore.persist(this._box, "checked");
-      } else {
-        xulStore.removeValue(document.documentURI, "sidebar-box", "checked");
-      }
-
-      xulStore.persist(this._box, "style");
-      xulStore.persist(this._title, "value");
-    }
-
-    Services.obs.removeObserver(this, "intl:app-locales-changed");
-
-    if (this._observer) {
-      this._observer.disconnect();
-      this._observer = null;
-    }
-  },
-
-  /**
-   * The handler for Services.obs.addObserver.
-   **/
-  observe(_subject, topic, _data) {
-    switch (topic) {
-      case "intl:app-locales-changed": {
-        if (this.isOpen) {
-          // The <tree> component used in history and bookmarks, but it does not
-          // support live switching the app locale. Reload the entire sidebar to
-          // invalidate any old text.
-          this.hide();
-          this.showInitially(this.lastOpenedId);
-          break;
-        }
-      }
-    }
-  },
-
-  /**
-   * Ensure the title stays in sync with the source element, which updates for
-   * l10n changes.
-   *
-   * @param {HTMLElement} [element]
-   */
-  observeTitleChanges(element) {
-    if (!element) {
-      return;
-    }
-    let observer = this._observer;
-    if (!observer) {
-      observer = new MutationObserver(() => {
-        this.title = this.sidebars.get(this.lastOpenedId).title;
-      });
-      // Re-use the observer.
-      this._observer = observer;
-    }
-    observer.disconnect();
-    observer.observe(element, {
-      attributes: true,
-      attributeFilter: ["label"],
-    });
-  },
-
-  /**
-   * Opens the switcher panel if it's closed, or closes it if it's open.
-   */
-  toggleSwitcherPanel() {
-    if (
-      this._switcherPanel.state == "open" ||
-      this._switcherPanel.state == "showing"
-    ) {
-      this.hideSwitcherPanel();
-    } else if (this._switcherPanel.state == "closed") {
-      this.showSwitcherPanel();
-    }
-  },
-
-  /**
-   * Handles keydown on the the switcherTarget button
-   * @param  {Event} event
-   */
-  handleKeydown(event) {
-    switch (event.key) {
-      case "Enter":
-      case " ": {
-        this.toggleSwitcherPanel();
-        event.stopPropagation();
-        event.preventDefault();
-        break;
-      }
-      case "Escape": {
-        this.hideSwitcherPanel();
-        event.stopPropagation();
-        event.preventDefault();
-        break;
-      }
-    }
-  },
-
-  hideSwitcherPanel() {
-    this._switcherPanel.hidePopup();
-  },
-
-  showSwitcherPanel() {
-    this._switcherPanel.addEventListener(
-      "popuphiding",
-      () => {
-        this._switcherTarget.classList.remove("active");
-        this._switcherTarget.setAttribute("aria-expanded", false);
-      },
-      { once: true }
-    );
-
-    // Combine start/end position with ltr/rtl to set the label in the popup appropriately.
-    let label =
-      this._positionStart == RTL_UI
-        ? gNavigatorBundle.getString("sidebar.moveToLeft")
-        : gNavigatorBundle.getString("sidebar.moveToRight");
-    this._reversePositionButton.setAttribute("label", label);
-
-    // Open the sidebar switcher popup, anchored off the switcher toggle
-    this._switcherPanel.hidden = false;
-    this._switcherPanel.openPopup(this._switcherTarget);
-
-    this._switcherTarget.classList.add("active");
-    this._switcherTarget.setAttribute("aria-expanded", true);
-  },
-
-  updateShortcut({ keyId }) {
-    let menuitem = this._switcherPanel?.querySelector(`[key="${keyId}"]`);
-    if (!menuitem) {
-      // If the menu item doesn't exist yet then the accel text will be set correctly
-      // upon creation so there's nothing to do now.
-      return;
-    }
-    menuitem.removeAttribute("acceltext");
-  },
-
-  /**
-   * Change the pref that will trigger a call to setPosition
-   */
-  reversePosition() {
-    Services.prefs.setBoolPref(this.POSITION_START_PREF, !this._positionStart);
-  },
-
-  /**
-   * Read the positioning pref and position the sidebar and the splitter
-   * appropriately within the browser container.
-   */
-  setPosition() {
-    // First reset all ordinals to match DOM ordering.
-    let browser = document.getElementById("browser");
-    [...browser.children].forEach((node, i) => {
-      node.style.order = i + 1;
-    });
-
-    if (!this._positionStart) {
-      // DOM ordering is:     |  sidebar-box  | splitter |   appcontent  |
-      // Want to display as:  |   appcontent  | splitter |  sidebar-box  |
-      // So we just swap box and appcontent ordering
-      let appcontent = document.getElementById("appcontent");
-      let boxOrdinal = this._box.style.order;
-      this._box.style.order = appcontent.style.order;
-      appcontent.style.order = boxOrdinal;
-      // Indicate we've switched ordering to the box
-      this._box.setAttribute("positionend", true);
-    } else {
-      this._box.removeAttribute("positionend");
-    }
-
-    this.hideSwitcherPanel();
-
-    let content = SidebarUI.browser.contentWindow;
-    if (content && content.updatePosition) {
-      content.updatePosition();
-    }
-  },
-
-  /**
-   * Try and adopt the status of the sidebar from another window.
-   * @param {Window} sourceWindow - Window to use as a source for sidebar status.
-   * @return true if we adopted the state, or false if the caller should
-   * initialize the state itself.
-   */
-  adoptFromWindow(sourceWindow) {
-    // If the opener had a sidebar, open the same sidebar in our window.
-    // The opener can be the hidden window too, if we're coming from the state
-    // where no windows are open, and the hidden window has no sidebar box.
-    let sourceUI = sourceWindow.SidebarUI;
-    if (!sourceUI || !sourceUI._box) {
-      // no source UI or no _box means we also can't adopt the state.
-      return false;
-    }
-
-    // Set sidebar command even if hidden, so that we keep the same sidebar
-    // even if it's currently closed.
-    let commandID = sourceUI._box.getAttribute("sidebarcommand");
-    if (commandID) {
-      this._box.setAttribute("sidebarcommand", commandID);
-    }
-
-    if (sourceUI._box.hidden) {
-      // just hidden means we have adopted the hidden state.
-      return true;
-    }
-
-    // dynamically generated sidebars will fail this check, but we still
-    // consider it adopted.
-    if (!this.sidebars.has(commandID)) {
-      return true;
-    }
-
-    this._box.style.width = sourceUI._box.getBoundingClientRect().width + "px";
-    this.showInitially(commandID);
-
-    return true;
-  },
-
-  windowPrivacyMatches(w1, w2) {
-    return (
-      PrivateBrowsingUtils.isWindowPrivate(w1) ===
-      PrivateBrowsingUtils.isWindowPrivate(w2)
-    );
-  },
-
-  /**
-   * If loading a sidebar was delayed on startup, start the load now.
-   */
-  startDelayedLoad() {
-    let sourceWindow = window.opener;
-    // No source window means this is the initial window.  If we're being
-    // opened from another window, check that it is one we might open a sidebar
-    // for.
-    if (sourceWindow) {
-      if (
-        sourceWindow.closed ||
-        sourceWindow.location.protocol != "chrome:" ||
-        !this.windowPrivacyMatches(sourceWindow, window)
-      ) {
-        return;
-      }
-      // Try to adopt the sidebar state from the source window
-      if (this.adoptFromWindow(sourceWindow)) {
-        return;
-      }
-    }
-
-    // If we're not adopting settings from a parent window, set them now.
-    let wasOpen = this._box.getAttribute("checked");
-    if (!wasOpen) {
-      return;
-    }
-
-    let commandID = this._box.getAttribute("sidebarcommand");
-    if (commandID && this.sidebars.has(commandID)) {
-      this.showInitially(commandID);
-    } else {
-      this._box.removeAttribute("checked");
-      // Remove the |sidebarcommand| attribute, because the element it
-      // refers to no longer exists, so we should assume this sidebar
-      // panel has been uninstalled. (249883)
-      // We use setAttribute rather than removeAttribute so it persists
-      // correctly.
-      this._box.setAttribute("sidebarcommand", "");
-      // On a startup in which the startup cache was invalidated (e.g. app update)
-      // extensions will not be started prior to delayedLoad, thus the
-      // sidebarcommand element will not exist yet.  Store the commandID so
-      // extensions may reopen if necessary.  A startup cache invalidation
-      // can be forced (for testing) by deleting compatibility.ini from the
-      // profile.
-      this.lastOpenedId = commandID;
-    }
-  },
-
-  /**
-   * Fire a "SidebarShown" event on the sidebar to give any interested parties
-   * a chance to update the button or whatever.
-   */
-  _fireShowEvent() {
-    let event = new CustomEvent("SidebarShown", { bubbles: true });
-    this._switcherTarget.dispatchEvent(event);
-  },
-
-  /**
-   * Fire a "SidebarFocused" event on the sidebar's |window| to give the sidebar
-   * a chance to adjust focus as needed. An additional event is needed, because
-   * we don't want to focus the sidebar when it's opened on startup or in a new
-   * window, only when the user opens the sidebar.
-   */
-  _fireFocusedEvent() {
-    let event = new CustomEvent("SidebarFocused", { bubbles: true });
-    this.browser.contentWindow.dispatchEvent(event);
-  },
-
-  /**
-   * True if the sidebar is currently open.
-   */
-  get isOpen() {
-    return !this._box.hidden;
-  },
-
-  /**
-   * The ID of the current sidebar.
-   */
-  get currentID() {
-    return this.isOpen ? this._box.getAttribute("sidebarcommand") : "";
-  },
-
-  get title() {
-    return this._title.value;
-  },
-
-  set title(value) {
-    this._title.value = value;
-  },
-
-  /**
-   * Toggle the visibility of the sidebar. If the sidebar is hidden or is open
-   * with a different commandID, then the sidebar will be opened using the
-   * specified commandID. Otherwise the sidebar will be hidden.
-   *
-   * @param  {string}  commandID     ID of the sidebar.
-   * @param  {DOMNode} [triggerNode] Node, usually a button, that triggered the
-   *                                 visibility toggling of the sidebar.
-   * @return {Promise}
-   */
-  toggle(commandID = this.lastOpenedId, triggerNode) {
-    if (
-      CustomizationHandler.isCustomizing() ||
-      CustomizationHandler.isExitingCustomizeMode
-    ) {
-      return Promise.resolve();
-    }
-    // First priority for a default value is this.lastOpenedId which is set during show()
-    // and not reset in hide(), unlike currentID. If show() hasn't been called and we don't
-    // have a persisted command either, or the command doesn't exist anymore, then
-    // fallback to a default sidebar.
-    if (!commandID) {
-      commandID = this._box.getAttribute("sidebarcommand");
-    }
-    if (!commandID || !this.sidebars.has(commandID)) {
-      commandID = this.DEFAULT_SIDEBAR_ID;
-    }
-
-    if (this.isOpen && commandID == this.currentID) {
-      this.hide(triggerNode);
-      return Promise.resolve();
-    }
-    return this.show(commandID, triggerNode);
-  },
-
-  _loadSidebarExtension(commandID) {
-    let sidebar = this.sidebars.get(commandID);
-    let { extensionId } = sidebar;
-    if (extensionId) {
-      SidebarUI.browser.contentWindow.loadPanel(
-        extensionId,
-        sidebar.panel,
-        sidebar.browserStyle
-      );
-    }
-  },
-
-  /**
-   * Show the sidebar.
-   *
-   * This wraps the internal method, including a ping to telemetry.
-   *
-   * @param {string}  commandID     ID of the sidebar to use.
-   * @param {DOMNode} [triggerNode] Node, usually a button, that triggered the
-   *                                showing of the sidebar.
-   * @return {Promise<boolean>}
-   */
-  async show(commandID, triggerNode) {
-    let panelType = commandID.substring(4, commandID.length - 7);
-    Services.telemetry.keyedScalarAdd("sidebar.opened", panelType, 1);
-
-    // Extensions without private window access wont be in the
-    // sidebars map.
-    if (!this.sidebars.has(commandID)) {
-      return false;
-    }
-    return this._show(commandID).then(() => {
-      this._loadSidebarExtension(commandID);
-
-      if (triggerNode) {
-        updateToggleControlLabel(triggerNode);
-      }
-
-      this._fireFocusedEvent();
-      return true;
-    });
-  },
-
-  /**
-   * Show the sidebar, without firing the focused event or logging telemetry.
-   * This is intended to be used when the sidebar is opened automatically
-   * when a window opens (not triggered by user interaction).
-   *
-   * @param {string} commandID ID of the sidebar.
-   * @return {Promise<boolean>}
-   */
-  async showInitially(commandID) {
-    let panelType = commandID.substring(4, commandID.length - 7);
-    Services.telemetry.keyedScalarAdd("sidebar.opened", panelType, 1);
-
-    // Extensions without private window access wont be in the
-    // sidebars map.
-    if (!this.sidebars.has(commandID)) {
-      return false;
-    }
-    return this._show(commandID).then(() => {
-      this._loadSidebarExtension(commandID);
-      return true;
-    });
-  },
-
-  /**
-   * Implementation for show. Also used internally for sidebars that are shown
-   * when a window is opened and we don't want to ping telemetry.
-   *
-   * @param {string} commandID ID of the sidebar.
-   * @return {Promise<void>}
-   */
-  _show(commandID) {
-    return new Promise(resolve => {
-      this.selectMenuItem(commandID);
-
-      this._box.hidden = this._splitter.hidden = false;
-      this.setPosition();
-
-      this.hideSwitcherPanel();
-
-      this._box.setAttribute("checked", "true");
-      this._box.setAttribute("sidebarcommand", commandID);
-      this.lastOpenedId = commandID;
-
-      let { url, title, sourceL10nEl } = this.sidebars.get(commandID);
-      this.title = title;
-      // Keep the title element in sync with any l10n changes.
-      this.observeTitleChanges(sourceL10nEl);
-      this.browser.setAttribute("src", url); // kick off async load
-
-      if (this.browser.contentDocument.location.href != url) {
-        this.browser.addEventListener(
-          "load",
-          event => {
-            // We're handling the 'load' event before it bubbles up to the usual
-            // (non-capturing) event handlers. Let it bubble up before resolving.
-            setTimeout(() => {
-              resolve();
-
-              // Now that the currentId is updated, fire a show event.
-              this._fireShowEvent();
-            }, 0);
-          },
-          { capture: true, once: true }
-        );
-      } else {
-        resolve();
-
-        // Now that the currentId is updated, fire a show event.
-        this._fireShowEvent();
-      }
-    });
-  },
-
-  /**
-   * Hide the sidebar.
-   *
-   * @param {DOMNode} [triggerNode] Node, usually a button, that triggered the
-   *                                hiding of the sidebar.
-   */
-  hide(triggerNode) {
-    if (!this.isOpen) {
-      return;
-    }
-
-    this.hideSwitcherPanel();
-
-    this.selectMenuItem("");
-
-    // Replace the document currently displayed in the sidebar with about:blank
-    // so that we can free memory by unloading the page. We need to explicitly
-    // create a new content viewer because the old one doesn't get destroyed
-    // until about:blank has loaded (which does not happen as long as the
-    // element is hidden).
-    this.browser.setAttribute("src", "about:blank");
-    this.browser.docShell.createAboutBlankDocumentViewer(null, null);
-
-    this._box.removeAttribute("checked");
-    this._box.hidden = this._splitter.hidden = true;
-
-    let selBrowser = gBrowser.selectedBrowser;
-    selBrowser.focus();
-    if (triggerNode) {
-      updateToggleControlLabel(triggerNode);
-    }
-  },
-
-  /**
-   * Sets the checked state only on the menu items of the specified sidebar, or
-   * none if the argument is an empty string.
-   */
-  selectMenuItem(commandID) {
-    for (let [id, { menuId, triggerButtonId }] of this.sidebars) {
-      let menu = document.getElementById(menuId);
-      let triggerbutton =
-        triggerButtonId && document.getElementById(triggerButtonId);
-      if (id == commandID) {
-        menu.setAttribute("checked", "true");
-        if (triggerbutton) {
-          triggerbutton.setAttribute("checked", "true");
-          updateToggleControlLabel(triggerbutton);
-        }
-      } else {
-        menu.removeAttribute("checked");
-        if (triggerbutton) {
-          triggerbutton.removeAttribute("checked");
-          updateToggleControlLabel(triggerbutton);
-        }
-      }
-    }
-  },
-};
-
-// Add getters related to the position here, since we will want them
-// available for both startDelayedLoad and init.
-XPCOMUtils.defineLazyPreferenceGetter(
-  SidebarUI,
-  "_positionStart",
-  SidebarUI.POSITION_START_PREF,
-  true,
-  SidebarUI.setPosition.bind(SidebarUI)
-);
diff --git a/browser/base/content/browser-siteIdentity.js b/browser/base/content/browser-siteIdentity.js
index a2a5f6ff71..60bd4fc01c 100644
--- a/browser/base/content/browser-siteIdentity.js
+++ b/browser/base/content/browser-siteIdentity.js
@@ -456,7 +456,9 @@ var gIdentityHandler = {
     );
 
     // Reload the page with the content unblocked
-    BrowserReloadWithFlags(Ci.nsIWebNavigation.LOAD_FLAGS_BYPASS_CACHE);
+    BrowserCommands.reloadWithFlags(
+      Ci.nsIWebNavigation.LOAD_FLAGS_BYPASS_CACHE
+    );
     if (this._popupInitialized) {
       PanelMultiView.hidePopup(this._identityPopup);
     }
@@ -475,7 +477,7 @@ var gIdentityHandler = {
       "mixed-content"
     );
     if (reload) {
-      BrowserReload();
+      BrowserCommands.reload();
     }
     if (this._popupInitialized) {
       PanelMultiView.hidePopup(this._identityPopup);
@@ -496,7 +498,7 @@ var gIdentityHandler = {
       port,
       gBrowser.contentPrincipal.originAttributes
     );
-    BrowserReloadSkipCache();
+    BrowserCommands.reloadSkipCache();
     if (this._popupInitialized) {
       PanelMultiView.hidePopup(this._identityPopup);
     }
@@ -611,7 +613,7 @@ var gIdentityHandler = {
     // Because "off" is 1 and "off temporarily" is 2, we can just check if the
     // sum of newValue and oldValue is 3.
     if (newValue + oldValue !== 3) {
-      BrowserReloadSkipCache();
+      BrowserCommands.reloadSkipCache();
       if (this._popupInitialized) {
         PanelMultiView.hidePopup(this._identityPopup);
       }
@@ -1260,7 +1262,7 @@ var gIdentityHandler = {
     }
   },
 
-  handleEvent(event) {
+  handleEvent() {
     let elem = document.activeElement;
     let position = elem.compareDocumentPosition(this._identityPopup);
 
@@ -1278,7 +1280,7 @@ var gIdentityHandler = {
     }
   },
 
-  observe(subject, topic, data) {
+  observe(subject, topic) {
     switch (topic) {
       case "perm-changed": {
         // Exclude permissions which do not appear in the UI in order to avoid
diff --git a/browser/base/content/browser-sitePermissionPanel.js b/browser/base/content/browser-sitePermissionPanel.js
index d81b636668..d977653f51 100644
--- a/browser/base/content/browser-sitePermissionPanel.js
+++ b/browser/base/content/browser-sitePermissionPanel.js
@@ -353,7 +353,7 @@ var gPermissionPanel = {
     }
   },
 
-  handleEvent(event) {
+  handleEvent() {
     let elem = document.activeElement;
     let position = elem.compareDocumentPosition(this._permissionPopup);
 
@@ -371,7 +371,7 @@ var gPermissionPanel = {
     }
   },
 
-  observe(subject, topic, data) {
+  observe(subject, topic) {
     switch (topic) {
       case "fullscreen-painted": {
         if (subject != window || !this._exitedEventReceived) {
diff --git a/browser/base/content/browser-siteProtections.js b/browser/base/content/browser-siteProtections.js
index c44b4d3e8e..043dc53b95 100644
--- a/browser/base/content/browser-siteProtections.js
+++ b/browser/base/content/browser-siteProtections.js
@@ -31,8 +31,6 @@ class ProtectionCategory {
    * @param {Object} options - Category options.
    * @param {string} options.prefEnabled - ID of pref which controls the
    * category enabled state.
-   * @param {string} [options.l10nId] - Identifier l10n strings are keyed under
-   * for this category. Defaults to protection ID.
    * @param {Object} flags - Flags for this category to look for in the content
    * blocking event and content blocking log.
    * @param {Number} [flags.load] - Load flag for this protection category. If
@@ -49,7 +47,7 @@ class ProtectionCategory {
    */
   constructor(
     id,
-    { prefEnabled, l10nId },
+    { prefEnabled },
     {
       load,
       block,
@@ -416,7 +414,6 @@ let TrackingProtection =
       super(
         "trackers",
         {
-          l10nId: "trackingContent",
           prefEnabled: "privacy.trackingprotection.enabled",
         },
         {
@@ -1065,7 +1062,6 @@ let SocialTracking =
       super(
         "socialblock",
         {
-          l10nId: "socialMediaTrackers",
           prefEnabled: "privacy.socialtracking.block_cookies.enabled",
         },
         {
@@ -1641,42 +1637,42 @@ var gProtectionsHandler = {
     );
   },
 
-  async showTrackersSubview(event) {
+  async showTrackersSubview() {
     await TrackingProtection.updateSubView();
     this._protectionsPopupMultiView.showSubView(
       "protections-popup-trackersView"
     );
   },
 
-  async showSocialblockerSubview(event) {
+  async showSocialblockerSubview() {
     await SocialTracking.updateSubView();
     this._protectionsPopupMultiView.showSubView(
       "protections-popup-socialblockView"
     );
   },
 
-  async showCookiesSubview(event) {
+  async showCookiesSubview() {
     await ThirdPartyCookies.updateSubView();
     this._protectionsPopupMultiView.showSubView(
       "protections-popup-cookiesView"
     );
   },
 
-  async showFingerprintersSubview(event) {
+  async showFingerprintersSubview() {
     await Fingerprinting.updateSubView();
     this._protectionsPopupMultiView.showSubView(
       "protections-popup-fingerprintersView"
     );
   },
 
-  async showCryptominersSubview(event) {
+  async showCryptominersSubview() {
     await Cryptomining.updateSubView();
     this._protectionsPopupMultiView.showSubView(
       "protections-popup-cryptominersView"
     );
   },
 
-  async onCookieBannerClick(event) {
+  async onCookieBannerClick() {
     if (!cookieBannerHandling.isSiteSupported) {
       return;
     }
@@ -2055,7 +2051,7 @@ var gProtectionsHandler = {
     }
   },
 
-  observe(subject, topic, data) {
+  observe(subject, topic) {
     switch (topic) {
       case "browser:purge-session-history":
         // We need to update the earliest recorded date if history has been
@@ -2194,7 +2190,7 @@ var gProtectionsHandler = {
     ContentBlockingAllowList.add(gBrowser.selectedBrowser);
     if (shouldReload) {
       this._hidePopup();
-      BrowserReload();
+      BrowserCommands.reload();
     }
   },
 
@@ -2202,11 +2198,11 @@ var gProtectionsHandler = {
     ContentBlockingAllowList.remove(gBrowser.selectedBrowser);
     if (shouldReload) {
       this._hidePopup();
-      BrowserReload();
+      BrowserCommands.reload();
     }
   },
 
-  async onTPSwitchCommand(event) {
+  async onTPSwitchCommand() {
     // When the switch is clicked, we wait 500ms and then disable/enable
     // protections, causing the page to refresh, and close the popup.
     // We need to ensure we don't handle more clicks during the 500ms delay,
@@ -2533,12 +2529,12 @@ var gProtectionsHandler = {
     };
 
     const doc = event.target.ownerDocument;
-    const container = doc.getElementById("messaging-system-message-container");
+    const container = doc.getElementById("info-message-container");
     const infoButton = doc.getElementById("protections-popup-info-button");
     const panelContainer = doc.getElementById("protections-popup");
     const toggleMessage = () => {
       const learnMoreLink = doc.querySelector(
-        "#messaging-system-message-container .text-link"
+        "#info-message-container .text-link"
       );
       if (learnMoreLink) {
         container.toggleAttribute("disabled");
diff --git a/browser/base/content/browser-sync.js b/browser/base/content/browser-sync.js
index a94bf2b896..065546d7b8 100644
--- a/browser/base/content/browser-sync.js
+++ b/browser/base/content/browser-sync.js
@@ -55,7 +55,7 @@ this.SyncedTabsPanelList = class SyncedTabsPanelList {
     this.createSyncedTabs();
   }
 
-  observe(subject, topic, data) {
+  observe(subject, topic) {
     if (topic == SyncedTabs.TOPIC_TABS_CHANGED) {
       this._showSyncedTabs();
     }
@@ -648,7 +648,7 @@ var gSync = {
   // shows the device list will start with `recentDeviceList`, but should also
   // force a refresh, both of which should mean in the worst-case, the UI is up
   // to date after a very short delay.
-  async ensureFxaDevices(options) {
+  async ensureFxaDevices() {
     if (UIState.get().status != UIState.STATUS_SIGNED_IN) {
       console.info("Skipping device list refresh; not signed in");
       return;
@@ -768,7 +768,7 @@ var gSync = {
           }
         }
 
-        item.addEventListener("command", event => {
+        item.addEventListener("command", () => {
           if (panelNode) {
             PanelMultiView.hidePopup(panelNode);
           }
@@ -1364,7 +1364,7 @@ var gSync = {
       return;
     }
     if (!createDeviceNodeFn) {
-      createDeviceNodeFn = (targetId, name, targetType, lastModified) => {
+      createDeviceNodeFn = (targetId, name) => {
         let eltName = name ? "menuitem" : "menuseparator";
         return document.createXULElement(eltName);
       };
@@ -1462,7 +1462,7 @@ var gSync = {
         fxAccounts.flushLogFile();
       });
     };
-    const onSendAllCommand = event => {
+    const onSendAllCommand = () => {
       send(targets);
     };
     const onTargetDeviceCommand = event => {
diff --git a/browser/base/content/browser-tabsintitlebar.js b/browser/base/content/browser-tabsintitlebar.js
index caf9986b2f..d7f71ed450 100644
--- a/browser/base/content/browser-tabsintitlebar.js
+++ b/browser/base/content/browser-tabsintitlebar.js
@@ -43,7 +43,7 @@ var TabsInTitlebar = {
     return document.documentElement.getAttribute("tabsintitlebar") == "true";
   },
 
-  observe(subject, topic, data) {
+  observe(subject, topic) {
     if (topic == "nsPref:changed") {
       this._readPref();
     }
diff --git a/browser/base/content/browser-thumbnails.js b/browser/base/content/browser-thumbnails.js
index 1162914ddf..2ca6148b67 100644
--- a/browser/base/content/browser-thumbnails.js
+++ b/browser/base/content/browser-thumbnails.js
@@ -103,7 +103,7 @@ var gBrowserThumbnails = {
     ChromeUtils.defineLazyGetter(this, "_topSiteURLs", getTopSiteURLs);
   },
 
-  notify: function Thumbnails_notify(timer) {
+  notify: function Thumbnails_notify() {
     gBrowserThumbnails._topSiteURLsRefreshTimer = null;
     gBrowserThumbnails.clearTopSiteURLCache();
   },
@@ -116,7 +116,7 @@ var gBrowserThumbnails = {
     aWebProgress,
     aRequest,
     aStateFlags,
-    aStatus
+    _aStatus
   ) {
     if (
       aStateFlags & Ci.nsIWebProgressListener.STATE_STOP &&
diff --git a/browser/base/content/browser-toolbarKeyNav.js b/browser/base/content/browser-toolbarKeyNav.js
index caa01100c5..c65d99f6f0 100644
--- a/browser/base/content/browser-toolbarKeyNav.js
+++ b/browser/base/content/browser-toolbarKeyNav.js
@@ -137,7 +137,7 @@ ToolbarKeyboardNavigator = {
   },
 
   // CustomizableUI event handler
-  onWidgetAdded(aWidgetId, aArea, aPosition) {
+  onWidgetAdded(aWidgetId, aArea) {
     if (!this.kToolbars.includes(aArea)) {
       return;
     }
diff --git a/browser/base/content/browser.css b/browser/base/content/browser.css
index c9ebddb7f5..6e776a9ce7 100644
--- a/browser/base/content/browser.css
+++ b/browser/base/content/browser.css
@@ -250,8 +250,7 @@ toolbar[customizing] > .overflow-button {
   display: none;
 }
 
-toolbar[customizing] #ion-button,
-toolbar[customizing] #whats-new-menu-button {
+toolbar[customizing] #ion-button {
   display: none;
 }
 
@@ -382,7 +381,7 @@ toolbarpaletteitem {
   toolbar[brighttext] & {
     list-style-image: var(--webextension-toolbar-image-light, inherit);
   }
-  toolbar:not([brighttext]) &:-moz-lwtheme {
+  :root[lwtheme] toolbar:not([brighttext]) & {
     list-style-image: var(--webextension-toolbar-image-dark, inherit);
   }
   toolbaritem:is([overflowedItem="true"], [cui-areatype="panel"]) > & {
@@ -480,12 +479,6 @@ toolbar:not(#TabsToolbar) > #personal-bookmarks {
   flex: 1;
 }
 
-@media (-moz-platform: macos) {
-  :root[inFullscreen="true"] {
-    padding-top: 0; /* override drawintitlebar="true" */
-  }
-}
-
 /* Hide menu elements intended for keyboard access support */
 #main-menubar[openedwithkey=false] .show-only-for-keyboard {
   display: none;
@@ -916,7 +909,7 @@ menupopup[emptyplacesresult="true"] > .hide-if-empty-places-result {
   position: absolute;
 }
 
-browser[tabmodalPromptShowing], browser[tabDialogShowing] {
+browser[tabDialogShowing] {
   -moz-user-focus: none !important;
 }
 
diff --git a/browser/base/content/browser.js b/browser/base/content/browser.js
index c91a5d4db2..72753da622 100644
--- a/browser/base/content/browser.js
+++ b/browser/base/content/browser.js
@@ -79,7 +79,6 @@ ChromeUtils.defineESModuleGetters(this, {
   SubDialog: "resource://gre/modules/SubDialog.sys.mjs",
   SubDialogManager: "resource://gre/modules/SubDialog.sys.mjs",
   TabCrashHandler: "resource:///modules/ContentCrashHandlers.sys.mjs",
-  TabModalPrompt: "chrome://global/content/tabprompts.sys.mjs",
   TabsSetupFlowManager:
     "resource:///modules/firefox-view-tabs-setup-manager.sys.mjs",
   TelemetryEnvironment: "resource://gre/modules/TelemetryEnvironment.sys.mjs",
@@ -107,6 +106,12 @@ ChromeUtils.defineLazyGetter(this, "fxAccounts", () => {
   ).getFxAccountsSingleton();
 });
 
+XPCOMUtils.defineLazyScriptGetter(
+  this,
+  ["BrowserCommands", "kSkipCacheFlags"],
+  "chrome://browser/content/browser-commands.js"
+);
+
 XPCOMUtils.defineLazyScriptGetter(
   this,
   "PlacesTreeView",
@@ -546,7 +551,7 @@ XPCOMUtils.defineLazyPreferenceGetter(
   "gFxaToolbarAccessed",
   "identity.fxaccounts.toolbar.accessed",
   false,
-  (aPref, aOldVal, aNewVal) => {
+  () => {
     updateFxaToolbarMenu(gFxaToolbarEnabled);
   }
 );
@@ -691,7 +696,6 @@ function shouldSuppressPopupNotifications() {
   // don't cover up the prompt.
   return (
     window.windowState == window.STATE_MINIMIZED ||
-    gBrowser?.selectedBrowser.hasAttribute("tabmodalChromePromptShowing") ||
     gBrowser?.selectedBrowser.hasAttribute("tabDialogShowing") ||
     gDialogBox?.isOpen
   );
@@ -1019,7 +1023,7 @@ const gClickAndHoldListenersOnElement = {
 };
 
 const gSessionHistoryObserver = {
-  observe(subject, topic, data) {
+  observe(subject, topic) {
     if (topic != "browser:purge-session-history") {
       return;
     }
@@ -1037,7 +1041,7 @@ const gSessionHistoryObserver = {
 const gStoragePressureObserver = {
   _lastNotificationTime: -1,
 
-  async observe(subject, topic, data) {
+  async observe(subject, topic) {
     if (topic != "QuotaManager::StoragePressure") {
       return;
     }
@@ -1088,7 +1092,7 @@ const gStoragePressureObserver = {
       document.l10n.setAttributes(message, "space-alert-over-5gb-message2");
       buttons.push({
         "l10n-id": "space-alert-over-5gb-settings-button",
-        callback(notificationBar, button) {
+        callback() {
           // The advanced subpanes are only supported in the old organization, which will
           // be removed by bug 1349689.
           openPreferences("privacy-sitedata");
@@ -1475,7 +1479,7 @@ var gKeywordURIFixup = {
     );
   },
 
-  observe(fixupInfo, topic, data) {
+  observe(fixupInfo) {
     fixupInfo.QueryInterface(Ci.nsIURIFixupInfo);
 
     let browser = fixupInfo.consumer?.top?.embedderElement;
@@ -2350,13 +2354,6 @@ var gBrowserInit = {
       readSetting();
     });
 
-    scheduleIdleTask(() => {
-      // load the tab preview component
-      import("chrome://browser/content/tabpreview/tabpreview.mjs").catch(
-        console.error
-      );
-    });
-
     scheduleIdleTask(() => {
       // setup simple gestures support
       gGestureSupport.init(true);
@@ -2620,17 +2617,17 @@ gBrowserInit.idleTasksFinishedPromise = new Promise(resolve => {
 function HandleAppCommandEvent(evt) {
   switch (evt.command) {
     case "Back":
-      BrowserBack();
+      BrowserCommands.back();
       break;
     case "Forward":
-      BrowserForward();
+      BrowserCommands.forward();
       break;
     case "Reload":
-      BrowserReloadSkipCache();
+      BrowserCommands.reloadSkipCache();
       break;
     case "Stop":
       if (XULBrowserWindow.stopCommand.getAttribute("disabled") != "true") {
-        BrowserStop();
+        BrowserCommands.stop();
       }
       break;
     case "Search":
@@ -2640,13 +2637,13 @@ function HandleAppCommandEvent(evt) {
       SidebarUI.toggle("viewBookmarksSidebar");
       break;
     case "Home":
-      BrowserHome();
+      BrowserCommands.home();
       break;
     case "New":
-      BrowserOpenTab();
+      BrowserCommands.openTab();
       break;
     case "Close":
-      BrowserCloseTabOrWindow();
+      BrowserCommands.closeTabOrWindow();
       break;
     case "Find":
       gLazyFindCommand("onFindCommand");
@@ -2655,7 +2652,7 @@ function HandleAppCommandEvent(evt) {
       openHelpLink("firefox-help");
       break;
     case "Open":
-      BrowserOpenFileWindow();
+      BrowserCommands.openFileWindow();
       break;
     case "Print":
       PrintUtils.startPrintWindow(gBrowser.selectedBrowser.browsingContext);
@@ -2673,203 +2670,6 @@ function HandleAppCommandEvent(evt) {
   evt.preventDefault();
 }
 
-function gotoHistoryIndex(aEvent) {
-  aEvent = getRootEvent(aEvent);
-
-  let index = aEvent.target.getAttribute("index");
-  if (!index) {
-    return false;
-  }
-
-  let where = whereToOpenLink(aEvent);
-
-  if (where == "current") {
-    // Normal click. Go there in the current tab and update session history.
-
-    try {
-      gBrowser.gotoIndex(index);
-    } catch (ex) {
-      return false;
-    }
-    return true;
-  }
-  // Modified click. Go there in a new tab/window.
-
-  let historyindex = aEvent.target.getAttribute("historyindex");
-  duplicateTabIn(gBrowser.selectedTab, where, Number(historyindex));
-  return true;
-}
-
-function BrowserForward(aEvent) {
-  let where = whereToOpenLink(aEvent, false, true);
-
-  if (where == "current") {
-    try {
-      gBrowser.goForward();
-    } catch (ex) {}
-  } else {
-    duplicateTabIn(gBrowser.selectedTab, where, 1);
-  }
-}
-
-function BrowserBack(aEvent) {
-  let where = whereToOpenLink(aEvent, false, true);
-
-  if (where == "current") {
-    try {
-      gBrowser.goBack();
-    } catch (ex) {}
-  } else {
-    duplicateTabIn(gBrowser.selectedTab, where, -1);
-  }
-}
-
-function BrowserHandleBackspace() {
-  switch (Services.prefs.getIntPref("browser.backspace_action")) {
-    case 0:
-      BrowserBack();
-      break;
-    case 1:
-      goDoCommand("cmd_scrollPageUp");
-      break;
-  }
-}
-
-function BrowserHandleShiftBackspace() {
-  switch (Services.prefs.getIntPref("browser.backspace_action")) {
-    case 0:
-      BrowserForward();
-      break;
-    case 1:
-      goDoCommand("cmd_scrollPageDown");
-      break;
-  }
-}
-
-function BrowserStop() {
-  gBrowser.webNavigation.stop(Ci.nsIWebNavigation.STOP_ALL);
-}
-
-function BrowserReloadOrDuplicate(aEvent) {
-  aEvent = getRootEvent(aEvent);
-  let accelKeyPressed =
-    AppConstants.platform == "macosx" ? aEvent.metaKey : aEvent.ctrlKey;
-  var backgroundTabModifier = aEvent.button == 1 || accelKeyPressed;
-
-  if (aEvent.shiftKey && !backgroundTabModifier) {
-    BrowserReloadSkipCache();
-    return;
-  }
-
-  let where = whereToOpenLink(aEvent, false, true);
-  if (where == "current") {
-    BrowserReload();
-  } else {
-    duplicateTabIn(gBrowser.selectedTab, where);
-  }
-}
-
-function BrowserReload() {
-  if (gBrowser.currentURI.schemeIs("view-source")) {
-    // Bug 1167797: For view source, we always skip the cache
-    return BrowserReloadSkipCache();
-  }
-  const reloadFlags = Ci.nsIWebNavigation.LOAD_FLAGS_NONE;
-  BrowserReloadWithFlags(reloadFlags);
-}
-
-const kSkipCacheFlags =
-  Ci.nsIWebNavigation.LOAD_FLAGS_BYPASS_PROXY |
-  Ci.nsIWebNavigation.LOAD_FLAGS_BYPASS_CACHE;
-function BrowserReloadSkipCache() {
-  // Bypass proxy and cache.
-  BrowserReloadWithFlags(kSkipCacheFlags);
-}
-
-function BrowserHome(aEvent) {
-  if (aEvent && "button" in aEvent && aEvent.button == 2) {
-    // right-click: do nothing
-    return;
-  }
-
-  var homePage = HomePage.get(window);
-  var where = whereToOpenLink(aEvent, false, true);
-  var urls;
-  var notifyObservers;
-
-  // Don't load the home page in pinned or hidden tabs (e.g. Firefox View).
-  if (
-    where == "current" &&
-    (gBrowser?.selectedTab.pinned || gBrowser?.selectedTab.hidden)
-  ) {
-    where = "tab";
-  }
-
-  // openTrustedLinkIn in utilityOverlay.js doesn't handle loading multiple pages
-  switch (where) {
-    case "current":
-      // If we're going to load an initial page in the current tab as the
-      // home page, we set initialPageLoadedFromURLBar so that the URL
-      // bar is cleared properly (even during a remoteness flip).
-      if (isInitialPage(homePage)) {
-        gBrowser.selectedBrowser.initialPageLoadedFromUserAction = homePage;
-      }
-      loadOneOrMoreURIs(
-        homePage,
-        Services.scriptSecurityManager.getSystemPrincipal(),
-        null
-      );
-      if (isBlankPageURL(homePage)) {
-        gURLBar.select();
-      } else {
-        gBrowser.selectedBrowser.focus();
-      }
-      notifyObservers = true;
-      aEvent?.preventDefault();
-      break;
-    case "tabshifted":
-    case "tab":
-      urls = homePage.split("|");
-      var loadInBackground = Services.prefs.getBoolPref(
-        "browser.tabs.loadBookmarksInBackground",
-        false
-      );
-      // The homepage observer event should only be triggered when the homepage opens
-      // in the foreground. This is mostly to support the homepage changed by extension
-      // doorhanger which doesn't currently support background pages. This may change in
-      // bug 1438396.
-      notifyObservers = !loadInBackground;
-      gBrowser.loadTabs(urls, {
-        inBackground: loadInBackground,
-        triggeringPrincipal:
-          Services.scriptSecurityManager.getSystemPrincipal(),
-        csp: null,
-      });
-      if (!loadInBackground) {
-        if (isBlankPageURL(homePage)) {
-          gURLBar.select();
-        } else {
-          gBrowser.selectedBrowser.focus();
-        }
-      }
-      aEvent?.preventDefault();
-      break;
-    case "window":
-      // OpenBrowserWindow will trigger the observer event, so no need to do so here.
-      notifyObservers = false;
-      OpenBrowserWindow();
-      aEvent?.preventDefault();
-      break;
-  }
-  if (notifyObservers) {
-    // A notification for when a user has triggered their homepage. This is used
-    // to display a doorhanger explaining that an extension has modified the
-    // homepage, if necessary. Observers are only notified if the homepage
-    // becomes the active page.
-    Services.obs.notifyObservers(null, "browser-open-homepage-start");
-  }
-}
-
 function loadOneOrMoreURIs(aURIString, aTriggeringPrincipal, aCsp) {
   // we're not a browser window, pass the URI string to a new browser window
   if (window.location.href != AppConstants.BROWSER_CHROME_URL) {
@@ -2918,62 +2718,6 @@ function openLocation(event) {
   );
 }
 
-function BrowserOpenTab({ event, url } = {}) {
-  let werePassedURL = !!url;
-  url ??= BROWSER_NEW_TAB_URL;
-  let searchClipboard = gMiddleClickNewTabUsesPasteboard && event?.button == 1;
-
-  let relatedToCurrent = false;
-  let where = "tab";
-
-  if (event) {
-    where = whereToOpenLink(event, false, true);
-
-    switch (where) {
-      case "tab":
-      case "tabshifted":
-        // When accel-click or middle-click are used, open the new tab as
-        // related to the current tab.
-        relatedToCurrent = true;
-        break;
-      case "current":
-        where = "tab";
-        break;
-    }
-  }
-
-  // A notification intended to be useful for modular peformance tracking
-  // starting as close as is reasonably possible to the time when the user
-  // expressed the intent to open a new tab.  Since there are a lot of
-  // entry points, this won't catch every single tab created, but most
-  // initiated by the user should go through here.
-  //
-  // Note 1: This notification gets notified with a promise that resolves
-  //         with the linked browser when the tab gets created
-  // Note 2: This is also used to notify a user that an extension has changed
-  //         the New Tab page.
-  Services.obs.notifyObservers(
-    {
-      wrappedJSObject: new Promise(resolve => {
-        let options = {
-          relatedToCurrent,
-          resolveOnNewTabCreated: resolve,
-        };
-        if (!werePassedURL && searchClipboard) {
-          let clipboard = readFromClipboard();
-          clipboard = UrlbarUtils.stripUnsafeProtocolOnPaste(clipboard).trim();
-          if (clipboard) {
-            url = clipboard;
-            options.allowThirdPartyFixup = true;
-          }
-        }
-        openTrustedLinkIn(url, where, options);
-      }),
-    },
-    "browser-open-newtab-start"
-  );
-}
-
 var gLastOpenDirectory = {
   _lastDir: null,
   get path() {
@@ -3014,76 +2758,6 @@ var gLastOpenDirectory = {
   },
 };
 
-function BrowserOpenFileWindow() {
-  // Get filepicker component.
-  try {
-    const nsIFilePicker = Ci.nsIFilePicker;
-    let fp = Cc["@mozilla.org/filepicker;1"].createInstance(nsIFilePicker);
-    let fpCallback = function fpCallback_done(aResult) {
-      if (aResult == nsIFilePicker.returnOK) {
-        try {
-          if (fp.file) {
-            gLastOpenDirectory.path = fp.file.parent.QueryInterface(Ci.nsIFile);
-          }
-        } catch (ex) {}
-        openTrustedLinkIn(fp.fileURL.spec, "current");
-      }
-    };
-
-    fp.init(
-      window.browsingContext,
-      gNavigatorBundle.getString("openFile"),
-      nsIFilePicker.modeOpen
-    );
-    fp.appendFilters(
-      nsIFilePicker.filterAll |
-        nsIFilePicker.filterText |
-        nsIFilePicker.filterImages |
-        nsIFilePicker.filterXML |
-        nsIFilePicker.filterHTML |
-        nsIFilePicker.filterPDF
-    );
-    fp.displayDirectory = gLastOpenDirectory.path;
-    fp.open(fpCallback);
-  } catch (ex) {}
-}
-
-function BrowserCloseTabOrWindow(event) {
-  // If we're not a browser window, just close the window.
-  if (window.location.href != AppConstants.BROWSER_CHROME_URL) {
-    closeWindow(true);
-    return;
-  }
-
-  // In a multi-select context, close all selected tabs
-  if (gBrowser.multiSelectedTabsCount) {
-    gBrowser.removeMultiSelectedTabs();
-    return;
-  }
-
-  // Keyboard shortcuts that would close a tab that is pinned select the first
-  // unpinned tab instead.
-  if (
-    event &&
-    (event.ctrlKey || event.metaKey || event.altKey) &&
-    gBrowser.selectedTab.pinned
-  ) {
-    if (gBrowser.visibleTabs.length > gBrowser._numPinnedTabs) {
-      gBrowser.tabContainer.selectedIndex = gBrowser._numPinnedTabs;
-    }
-    return;
-  }
-
-  // If the current tab is the last one, this will close the window.
-  gBrowser.removeCurrentTab({ animate: true });
-}
-
-function BrowserTryToCloseWindow(event) {
-  if (WindowIsClosing(event)) {
-    window.close();
-  } // WindowIsClosing does all the necessary checks
-}
-
 function getLoadContext() {
   return window.docShell.QueryInterface(Ci.nsILoadContext);
 }
@@ -3120,162 +2794,6 @@ function readFromClipboard() {
   return url;
 }
 
-/**
- * Open the View Source dialog.
- *
- * @param args
- *        An object with the following properties:
- *
- *        URL (required):
- *          A string URL for the page we'd like to view the source of.
- *        browser (optional):
- *          The browser containing the document that we would like to view the
- *          source of. This is required if outerWindowID is passed.
- *        outerWindowID (optional):
- *          The outerWindowID of the content window containing the document that
- *          we want to view the source of. You only need to provide this if you
- *          want to attempt to retrieve the document source from the network
- *          cache.
- *        lineNumber (optional):
- *          The line number to focus on once the source is loaded.
- */
-async function BrowserViewSourceOfDocument(args) {
-  // Check if external view source is enabled.  If so, try it.  If it fails,
-  // fallback to internal view source.
-  if (Services.prefs.getBoolPref("view_source.editor.external")) {
-    try {
-      await top.gViewSourceUtils.openInExternalEditor(args);
-      return;
-    } catch (data) {}
-  }
-
-  let tabBrowser = gBrowser;
-  let preferredRemoteType;
-  let initialBrowsingContextGroupId;
-  if (args.browser) {
-    preferredRemoteType = args.browser.remoteType;
-    initialBrowsingContextGroupId = args.browser.browsingContext.group.id;
-  } else {
-    if (!tabBrowser) {
-      throw new Error(
-        "BrowserViewSourceOfDocument should be passed the " +
-          "subject browser if called from a window without " +
-          "gBrowser defined."
-      );
-    }
-    // Some internal URLs (such as specific chrome: and about: URLs that are
-    // not yet remote ready) cannot be loaded in a remote browser.  View
-    // source in tab expects the new view source browser's remoteness to match
-    // that of the original URL, so disable remoteness if necessary for this
-    // URL.
-    var oa = E10SUtils.predictOriginAttributes({ window });
-    preferredRemoteType = E10SUtils.getRemoteTypeForURI(
-      args.URL,
-      gMultiProcessBrowser,
-      gFissionBrowser,
-      E10SUtils.DEFAULT_REMOTE_TYPE,
-      null,
-      oa
-    );
-  }
-
-  // In the case of popups, we need to find a non-popup browser window.
-  if (!tabBrowser || !window.toolbar.visible) {
-    // This returns only non-popup browser windows by default.
-    let browserWindow = BrowserWindowTracker.getTopWindow();
-    tabBrowser = browserWindow.gBrowser;
-  }
-
-  const inNewWindow = !Services.prefs.getBoolPref("view_source.tab");
-
-  // `viewSourceInBrowser` will load the source content from the page
-  // descriptor for the tab (when possible) or fallback to the network if
-  // that fails.  Either way, the view source module will manage the tab's
-  // location, so use "about:blank" here to avoid unnecessary redundant
-  // requests.
-  let tab = tabBrowser.addTab("about:blank", {
-    relatedToCurrent: true,
-    inBackground: inNewWindow,
-    skipAnimation: inNewWindow,
-    preferredRemoteType,
-    initialBrowsingContextGroupId,
-    triggeringPrincipal: Services.scriptSecurityManager.getSystemPrincipal(),
-    skipLoad: true,
-  });
-  args.viewSourceBrowser = tabBrowser.getBrowserForTab(tab);
-  top.gViewSourceUtils.viewSourceInBrowser(args);
-
-  if (inNewWindow) {
-    tabBrowser.hideTab(tab);
-    tabBrowser.replaceTabWithWindow(tab);
-  }
-}
-
-/**
- * Opens the View Source dialog for the source loaded in the root
- * top-level document of the browser. This is really just a
- * convenience wrapper around BrowserViewSourceOfDocument.
- *
- * @param browser
- *        The browser that we want to load the source of.
- */
-function BrowserViewSource(browser) {
-  BrowserViewSourceOfDocument({
-    browser,
-    outerWindowID: browser.outerWindowID,
-    URL: browser.currentURI.spec,
-  });
-}
-
-// documentURL - URL of the document to view, or null for this window's document
-// initialTab - name of the initial tab to display, or null for the first tab
-// imageElement - image to load in the Media Tab of the Page Info window; can be null/omitted
-// browsingContext - the browsingContext of the frame that we want to view information about; can be null/omitted
-// browser - the browser containing the document we're interested in inspecting; can be null/omitted
-function BrowserPageInfo(
-  documentURL,
-  initialTab,
-  imageElement,
-  browsingContext,
-  browser
-) {
-  let args = { initialTab, imageElement, browsingContext, browser };
-
-  documentURL = documentURL || window.gBrowser.selectedBrowser.currentURI.spec;
-
-  let isPrivate = PrivateBrowsingUtils.isWindowPrivate(window);
-
-  // Check for windows matching the url
-  for (let currentWindow of Services.wm.getEnumerator("Browser:page-info")) {
-    if (currentWindow.closed) {
-      continue;
-    }
-    if (
-      currentWindow.document.documentElement.getAttribute("relatedUrl") ==
-        documentURL &&
-      PrivateBrowsingUtils.isWindowPrivate(currentWindow) == isPrivate
-    ) {
-      currentWindow.focus();
-      currentWindow.resetPageInfo(args);
-      return currentWindow;
-    }
-  }
-
-  // We didn't find a matching window, so open a new one.
-  let options = "chrome,toolbar,dialog=no,resizable";
-
-  // Ensure the window groups correctly in the Windows taskbar
-  if (isPrivate) {
-    options += ",private";
-  }
-  return openDialog(
-    "chrome://browser/content/pageinfo/pageInfo.xhtml",
-    "",
-    options,
-    args
-  );
-}
-
 function UpdateUrlbarSearchSplitterState() {
   var splitter = document.getElementById("urlbar-search-splitter");
   var urlbar = document.getElementById("urlbar-container");
@@ -3480,88 +2998,6 @@ function getDefaultHomePage() {
   return url;
 }
 
-function BrowserFullScreen() {
-  window.fullScreen = !window.fullScreen || BrowserHandler.kiosk;
-}
-
-function BrowserReloadWithFlags(reloadFlags) {
-  let unchangedRemoteness = [];
-
-  for (let tab of gBrowser.selectedTabs) {
-    let browser = tab.linkedBrowser;
-    let url = browser.currentURI;
-    let urlSpec = url.spec;
-    // We need to cache the content principal here because the browser will be
-    // reconstructed when the remoteness changes and the content prinicpal will
-    // be cleared after reconstruction.
-    let principal = tab.linkedBrowser.contentPrincipal;
-    if (gBrowser.updateBrowserRemotenessByURL(browser, urlSpec)) {
-      // If the remoteness has changed, the new browser doesn't have any
-      // information of what was loaded before, so we need to load the previous
-      // URL again.
-      if (tab.linkedPanel) {
-        loadBrowserURI(browser, url, principal);
-      } else {
-        // Shift to fully loaded browser and make
-        // sure load handler is instantiated.
-        tab.addEventListener(
-          "SSTabRestoring",
-          () => loadBrowserURI(browser, url, principal),
-          { once: true }
-        );
-        gBrowser._insertBrowser(tab);
-      }
-    } else {
-      unchangedRemoteness.push(tab);
-    }
-  }
-
-  if (!unchangedRemoteness.length) {
-    return;
-  }
-
-  // Reset temporary permissions on the remaining tabs to reload.
-  // This is done here because we only want to reset
-  // permissions on user reload.
-  for (let tab of unchangedRemoteness) {
-    SitePermissions.clearTemporaryBlockPermissions(tab.linkedBrowser);
-    // Also reset DOS mitigations for the basic auth prompt on reload.
-    delete tab.linkedBrowser.authPromptAbuseCounter;
-  }
-  gIdentityHandler.hidePopup();
-  gPermissionPanel.hidePopup();
-
-  let handlingUserInput = document.hasValidTransientUserGestureActivation;
-
-  for (let tab of unchangedRemoteness) {
-    if (tab.linkedPanel) {
-      sendReloadMessage(tab);
-    } else {
-      // Shift to fully loaded browser and make
-      // sure load handler is instantiated.
-      tab.addEventListener("SSTabRestoring", () => sendReloadMessage(tab), {
-        once: true,
-      });
-      gBrowser._insertBrowser(tab);
-    }
-  }
-
-  function loadBrowserURI(browser, url, principal) {
-    browser.loadURI(url, {
-      flags: reloadFlags,
-      triggeringPrincipal: principal,
-    });
-  }
-
-  function sendReloadMessage(tab) {
-    tab.linkedBrowser.sendMessageToActor(
-      "Browser:Reload",
-      { flags: reloadFlags, handlingUserInput },
-      "BrowserTab"
-    );
-  }
-}
-
 // TODO: can we pull getPEMString in from pippki.js instead of
 // duplicating them here?
 function getPEMString(cert) {
@@ -4053,7 +3489,7 @@ const BrowserSearch = {
         win.BrowserSearch.webSearch();
       } else {
         // If there are no open browser windows, open a new one
-        var observer = function (subject, topic, data) {
+        var observer = function (subject) {
           if (subject == win) {
             BrowserSearch.webSearch();
             Services.obs.removeObserver(
@@ -4444,7 +3880,8 @@ function FillHistoryMenu(aParent) {
       item.setAttribute("label", entry.title || uri);
       item.setAttribute("index", j);
 
-      // Cache this so that gotoHistoryIndex doesn't need the original index
+      // Cache this so that BrowserCommands.gotoHistoryIndex doesn't need the
+      // original index
       item.setAttribute("historyindex", j - index);
 
       if (j != index) {
@@ -5037,7 +4474,7 @@ var XULBrowserWindow = {
     this.setOverLink("", { hideStatusPanelImmediately: true });
   },
 
-  showTooltip(xDevPix, yDevPix, tooltip, direction, browser) {
+  showTooltip(xDevPix, yDevPix, tooltip, direction, _browser) {
     if (
       Cc["@mozilla.org/widget/dragservice;1"]
         .getService(Ci.nsIDragService)
@@ -5070,14 +4507,7 @@ var XULBrowserWindow = {
     return gBrowser.tabs.length;
   },
 
-  onProgressChange(
-    aWebProgress,
-    aRequest,
-    aCurSelfProgress,
-    aMaxSelfProgress,
-    aCurTotalProgress,
-    aMaxTotalProgress
-  ) {
+  onProgressChange() {
     // Do nothing.
   },
 
@@ -5576,7 +5006,7 @@ var XULBrowserWindow = {
   //  2. Called by tabbrowser.xml when updating the current browser.
   //  3. Called directly during this object's initializations.
   // aRequest will be null always in case 2 and 3, and sometimes in case 1.
-  onSecurityChange(aWebProgress, aRequest, aState, aIsSimulated) {
+  onSecurityChange(aWebProgress, aRequest, aState, _aIsSimulated) {
     // Don't need to do anything if the data we use to update the UI hasn't
     // changed
     let uri = gBrowser.currentURI;
@@ -5619,7 +5049,7 @@ var XULBrowserWindow = {
     aStateFlags,
     aStatus,
     aMessage,
-    aTotalProgress
+    _aTotalProgress
   ) {
     if (FullZoom.updateBackgroundTabs) {
       FullZoom.onLocationChange(gBrowser.currentURI, true);
@@ -6087,7 +5517,7 @@ nsBrowserAccess.prototype = {
     }
 
     if (aIsExternal && (!aURI || aURI.spec == "about:blank")) {
-      win.BrowserOpenTab(); // this also focuses the location bar
+      win.BrowserCommands.openTab(); // this also focuses the location bar
       win.focus();
       return win.gBrowser.selectedBrowser;
     }
@@ -6801,7 +6231,7 @@ var gTabletModePageCounter = {
 };
 
 function displaySecurityInfo() {
-  BrowserPageInfo(null, "securityTab");
+  BrowserCommands.pageInfo(null, "securityTab");
 }
 
 // Updates the UI density (for touch and compact mode) based on the uidensity pref.
@@ -7299,7 +6729,9 @@ function handleDroppedLink(
 
 function BrowserForceEncodingDetection() {
   gBrowser.selectedBrowser.forceEncodingDetection();
-  BrowserReloadWithFlags(Ci.nsIWebNavigation.LOAD_FLAGS_CHARSET_CHANGE);
+  BrowserCommands.reloadWithFlags(
+    Ci.nsIWebNavigation.LOAD_FLAGS_CHARSET_CHANGE
+  );
 }
 
 var ToolbarContextMenu = {
@@ -7467,7 +6899,7 @@ var BrowserOffline = {
   },
 
   // nsIObserver
-  observe(aSubject, aTopic, aState) {
+  observe(aSubject, aTopic) {
     if (aTopic != "network:offline-status-changed") {
       return;
     }
@@ -7695,8 +7127,8 @@ var WebAuthnPromptHelper = {
 
     if (data.prompt.type == "presence") {
       this.presence_required(mgr, data);
-    } else if (data.prompt.type == "register-direct") {
-      this.registerDirect(mgr, data);
+    } else if (data.prompt.type == "attestation-consent") {
+      this.attestation_consent(mgr, data);
     } else if (data.prompt.type == "pin-required") {
       this.pin_required(mgr, false, data);
     } else if (data.prompt.type == "pin-invalid") {
@@ -7815,7 +7247,7 @@ var WebAuthnPromptHelper = {
       secondaryActions.push({
         label,
         accessKey: i.toString(),
-        callback(aState) {
+        callback() {
           mgr.selectionCallback(tid, i);
         },
       });
@@ -7859,9 +7291,23 @@ var WebAuthnPromptHelper = {
     );
   },
 
-  registerDirect(mgr, { origin, tid }) {
-    let mainAction = this.buildProceedAction(mgr, tid);
-    let secondaryActions = [this.buildCancelAction(mgr, tid)];
+  attestation_consent(mgr, { origin, tid }) {
+    let mainAction = {
+      label: gNavigatorBundle.getString("webauthn.allow"),
+      accessKey: gNavigatorBundle.getString("webauthn.allow.accesskey"),
+      callback(_state) {
+        mgr.setHasAttestationConsent(tid, true);
+      },
+    };
+    let secondaryActions = [
+      {
+        label: gNavigatorBundle.getString("webauthn.block"),
+        accessKey: gNavigatorBundle.getString("webauthn.block.accesskey"),
+        callback(_state) {
+          mgr.setHasAttestationConsent(tid, false);
+        },
+      },
+    ];
 
     let learnMoreURL =
       Services.urlFormatter.formatURLPref("app.support.baseURL") +
@@ -7869,9 +7315,6 @@ var WebAuthnPromptHelper = {
 
     let options = {
       learnMoreURL,
-      checkbox: {
-        label: gNavigatorBundle.getString("webauthn.anonymize"),
-      },
       hintText: "webauthn.registerDirectPromptHint",
     };
     this.show(
@@ -7994,16 +7437,6 @@ var WebAuthnPromptHelper = {
     }
   },
 
-  buildProceedAction(mgr, tid) {
-    return {
-      label: gNavigatorBundle.getString("webauthn.proceed"),
-      accessKey: gNavigatorBundle.getString("webauthn.proceed.accesskey"),
-      callback(state) {
-        mgr.resumeMakeCredential(tid, state.checkboxChecked);
-      },
-    };
-  },
-
   buildCancelAction(mgr, tid) {
     return {
       label: gNavigatorBundle.getString("webauthn.cancel"),
@@ -8220,7 +7653,7 @@ function BrowserOpenAddonsMgr(aView, { selectTabByViewId = false } = {}) {
     let emWindow;
     let browserWindow;
 
-    var receivePong = function (aSubject, aTopic, aData) {
+    var receivePong = function (aSubject) {
       let browserWin = aSubject.browsingContext.topChromeWindow;
       if (!emWindow || browserWin == window /* favor the current window */) {
         if (
@@ -8262,7 +7695,7 @@ function BrowserOpenAddonsMgr(aView, { selectTabByViewId = false } = {}) {
       switchToTabHavingURI("about:addons", true);
     }
 
-    Services.obs.addObserver(function observer(aSubject, aTopic, aData) {
+    Services.obs.addObserver(function observer(aSubject, aTopic) {
       Services.obs.removeObserver(observer, aTopic);
       if (aView) {
         aSubject.loadView(aView);
@@ -8449,7 +7882,7 @@ function ReportSiteIssue() {
  * and when the "remote-listening" system notification fires.
  */
 const gRemoteControl = {
-  observe(subject, topic, data) {
+  observe() {
     gRemoteControl.updateVisualCue();
   },
 
@@ -8658,7 +8091,7 @@ function switchToTabHavingURI(
         ignoreQueryString || replaceQueryString,
         ignoreFragmentWhenComparing
       );
-      let browserUserContextId = browser.getAttribute("usercontextid");
+      let browserUserContextId = browser.getAttribute("usercontextid") || "";
       if (aUserContextId != null && aUserContextId != browserUserContextId) {
         continue;
       }
@@ -9447,221 +8880,6 @@ TabDialogBox.prototype.QueryInterface = ChromeUtils.generateQI([
   "nsISupportsWeakReference",
 ]);
 
-function TabModalPromptBox(browser) {
-  this._weakBrowserRef = Cu.getWeakReference(browser);
-  /*
-   * These WeakMaps holds the TabModalPrompt instances, key to the <tabmodalprompt> prompt
-   * in the DOM. We don't want to hold the instances directly to avoid leaking.
-   *
-   * WeakMap also prevents us from reading back its insertion order.
-   * Order of the elements in the DOM should be the only order to consider.
-   */
-  this._contentPrompts = new WeakMap();
-  this._tabPrompts = new WeakMap();
-}
-
-TabModalPromptBox.prototype = {
-  _promptCloseCallback(
-    onCloseCallback,
-    principalToAllowFocusFor,
-    allowFocusCheckbox,
-    ...args
-  ) {
-    if (
-      principalToAllowFocusFor &&
-      allowFocusCheckbox &&
-      allowFocusCheckbox.checked
-    ) {
-      Services.perms.addFromPrincipal(
-        principalToAllowFocusFor,
-        "focus-tab-by-prompt",
-        Services.perms.ALLOW_ACTION
-      );
-    }
-    onCloseCallback.apply(this, args);
-  },
-
-  getPrompt(promptEl) {
-    if (promptEl.classList.contains("tab-prompt")) {
-      return this._tabPrompts.get(promptEl);
-    }
-    return this._contentPrompts.get(promptEl);
-  },
-
-  appendPrompt(args, onCloseCallback) {
-    let browser = this.browser;
-    let newPrompt = new TabModalPrompt(browser.ownerGlobal);
-
-    if (args.modalType === Ci.nsIPrompt.MODAL_TYPE_TAB) {
-      newPrompt.element.classList.add("tab-prompt");
-      this._tabPrompts.set(newPrompt.element, newPrompt);
-    } else {
-      newPrompt.element.classList.add("content-prompt");
-      this._contentPrompts.set(newPrompt.element, newPrompt);
-    }
-
-    browser.parentNode.insertBefore(
-      newPrompt.element,
-      browser.nextElementSibling
-    );
-    browser.setAttribute("tabmodalPromptShowing", true);
-
-    // Indicate if a tab modal chrome prompt is being shown so that
-    // PopupNotifications are suppressed.
-    if (
-      args.modalType === Ci.nsIPrompt.MODAL_TYPE_TAB &&
-      !browser.hasAttribute("tabmodalChromePromptShowing")
-    ) {
-      browser.setAttribute("tabmodalChromePromptShowing", true);
-      // Notify popup notifications of the UI change so they hide their
-      // notification panels.
-      UpdatePopupNotificationsVisibility();
-    }
-
-    let prompts = this.listPrompts(args.modalType);
-    if (prompts.length > 1) {
-      // Let's hide ourself behind the current prompt.
-      newPrompt.element.hidden = true;
-    }
-
-    let principalToAllowFocusFor = this._allowTabFocusByPromptPrincipal;
-    delete this._allowTabFocusByPromptPrincipal;
-
-    let allowFocusCheckbox; // Define outside the if block so we can bind it into the callback.
-    let hostForAllowFocusCheckbox = "";
-    try {
-      hostForAllowFocusCheckbox = principalToAllowFocusFor.URI.host;
-    } catch (ex) {
-      /* Ignore exceptions for host-less URIs */
-    }
-    if (hostForAllowFocusCheckbox) {
-      let allowFocusRow = document.createElement("div");
-
-      let spacer = document.createElement("div");
-      allowFocusRow.appendChild(spacer);
-
-      allowFocusCheckbox = document.createXULElement("checkbox");
-      document.l10n.setAttributes(
-        allowFocusCheckbox,
-        "tabbrowser-allow-dialogs-to-get-focus",
-        { domain: hostForAllowFocusCheckbox }
-      );
-      allowFocusRow.appendChild(allowFocusCheckbox);
-
-      newPrompt.ui.rows.append(allowFocusRow);
-    }
-
-    let tab = gBrowser.getTabForBrowser(browser);
-    let closeCB = this._promptCloseCallback.bind(
-      null,
-      onCloseCallback,
-      principalToAllowFocusFor,
-      allowFocusCheckbox
-    );
-    newPrompt.init(args, tab, closeCB);
-    return newPrompt;
-  },
-
-  removePrompt(aPrompt) {
-    let { modalType } = aPrompt.args;
-    if (modalType === Ci.nsIPrompt.MODAL_TYPE_TAB) {
-      this._tabPrompts.delete(aPrompt.element);
-    } else {
-      this._contentPrompts.delete(aPrompt.element);
-    }
-
-    let browser = this.browser;
-    aPrompt.element.remove();
-
-    let prompts = this.listPrompts(modalType);
-    if (prompts.length) {
-      let prompt = prompts[prompts.length - 1];
-      prompt.element.hidden = false;
-      // Because we were hidden before, this won't have been possible, so do it now:
-      prompt.Dialog.setDefaultFocus();
-    } else if (modalType === Ci.nsIPrompt.MODAL_TYPE_TAB) {
-      // If we remove the last tab chrome prompt, also remove the browser
-      // attribute.
-      browser.removeAttribute("tabmodalChromePromptShowing");
-      // Notify popup notifications of the UI change so they show notification
-      // panels again.
-      UpdatePopupNotificationsVisibility();
-    }
-    // Check if all prompts are closed
-    if (!this._hasPrompts()) {
-      browser.removeAttribute("tabmodalPromptShowing");
-      browser.focus();
-    }
-  },
-
-  /**
-   * Checks if the prompt box has prompt elements.
-   * @returns {Boolean} - true if there are prompt elements.
-   */
-  _hasPrompts() {
-    return !!this._getPromptElements().length;
-  },
-
-  /**
-   * Get list of current prompt elements.
-   * @param {Number} [aModalType] - Optionally filter by
-   * Ci.nsIPrompt.MODAL_TYPE_.
-   * @returns {NodeList} - A list of tabmodalprompt elements.
-   */
-  _getPromptElements(aModalType = null) {
-    let selector = "tabmodalprompt";
-
-    if (aModalType != null) {
-      if (aModalType === Ci.nsIPrompt.MODAL_TYPE_TAB) {
-        selector += ".tab-prompt";
-      } else {
-        selector += ".content-prompt";
-      }
-    }
-    return this.browser.parentNode.querySelectorAll(selector);
-  },
-
-  /**
-   * Get a list of all TabModalPrompt objects associated with the prompt box.
-   * @param {Number} [aModalType] - Optionally filter by
-   * Ci.nsIPrompt.MODAL_TYPE_.
-   * @returns {TabModalPrompt[]} - An array of TabModalPrompt objects.
-   */
-  listPrompts(aModalType = null) {
-    // Get the nodelist, then return the TabModalPrompt instances as an array
-    let promptMap;
-
-    if (aModalType) {
-      if (aModalType === Ci.nsIPrompt.MODAL_TYPE_TAB) {
-        promptMap = this._tabPrompts;
-      } else {
-        promptMap = this._contentPrompts;
-      }
-    }
-
-    let elements = this._getPromptElements(aModalType);
-
-    if (promptMap) {
-      return [...elements].map(el => promptMap.get(el));
-    }
-    return [...elements].map(
-      el => this._contentPrompts.get(el) || this._tabPrompts.get(el)
-    );
-  },
-
-  onNextPromptShowAllowFocusCheckboxFor(principal) {
-    this._allowTabFocusByPromptPrincipal = principal;
-  },
-
-  get browser() {
-    let browser = this._weakBrowserRef.get();
-    if (!browser) {
-      throw new Error("Stale promptbox! The associated browser is gone.");
-    }
-    return browser;
-  },
-};
-
 // Handle window-modal prompts that we want to display with the same style as
 // tab-modal prompts.
 var gDialogBox = {
diff --git a/browser/base/content/browser.js.globals b/browser/base/content/browser.js.globals
index c767bb5beb..7002cd0b5b 100644
--- a/browser/base/content/browser.js.globals
+++ b/browser/base/content/browser.js.globals
@@ -32,37 +32,19 @@
   "delayedStartupPromise",
   "gBrowserInit",
   "HandleAppCommandEvent",
-  "gotoHistoryIndex",
-  "BrowserForward",
-  "BrowserBack",
-  "BrowserHandleBackspace",
-  "BrowserHandleShiftBackspace",
-  "BrowserStop",
-  "BrowserReloadOrDuplicate",
-  "BrowserReload",
+  "BrowserCommands",
   "kSkipCacheFlags",
-  "BrowserReloadSkipCache",
-  "BrowserHome",
   "loadOneOrMoreURIs",
   "openLocation",
-  "BrowserOpenTab",
   "gLastOpenDirectory",
-  "BrowserOpenFileWindow",
-  "BrowserCloseTabOrWindow",
-  "BrowserTryToCloseWindow",
   "getLoadContext",
   "readFromClipboard",
-  "BrowserViewSourceOfDocument",
-  "BrowserViewSource",
-  "BrowserPageInfo",
   "UpdateUrlbarSearchSplitterState",
   "UpdatePopupNotificationsVisibility",
   "PageProxyClickHandler",
   "BrowserOnClick",
   "getMeOutOfHere",
   "getDefaultHomePage",
-  "BrowserFullScreen",
-  "BrowserReloadWithFlags",
   "getPEMString",
   "browserDragAndDrop",
   "homeButtonObserver",
@@ -131,7 +113,6 @@
   "PanicButtonNotifier",
   "SafeBrowsingNotificationBox",
   "TabDialogBox",
-  "TabModalPromptBox",
   "gDialogBox",
   "ConfirmationHint",
   "FirefoxViewHandler",
@@ -196,7 +177,6 @@
   "SubDialog",
   "SubDialogManager",
   "TabCrashHandler",
-  "TabModalPrompt",
   "TabsSetupFlowManager",
   "TelemetryEnvironment",
   "TranslationsParent",
diff --git a/browser/base/content/browser.xhtml b/browser/base/content/browser.xhtml
index 1dcdd02cd1..a01138c7da 100644
--- a/browser/base/content/browser.xhtml
+++ b/browser/base/content/browser.xhtml
@@ -38,9 +38,6 @@
       both "content" and "skin" packages, which bug 1385444 will unify later. -->
   <link rel="stylesheet" href="chrome://global/skin/global.css" />
 
-  <link rel="stylesheet" href="chrome://global/content/tabprompts.css" />
-  <link rel="stylesheet" href="chrome://global/skin/tabprompts.css" />
-
   <link rel="stylesheet" href="chrome://browser/content/browser.css" />
   <link rel="stylesheet" href="chrome://browser/content/tabbrowser.css" />
   <link
@@ -98,6 +95,7 @@
   <link rel="localization" href="preview/select-translations.ftl"/>
   <link rel="localization" href="browser/shopping.ftl"/>
   <link rel="localization" href="preview/shopping.ftl"/>
+  <link rel="localization" href="preview/sidebar.ftl"/>
   <link rel="localization" href="preview/profiles.ftl"/>
   <link rel="localization" href="preview/onboarding.ftl"/>
 
@@ -119,7 +117,7 @@
     Services.scriptloader.loadSubScript("chrome://browser/content/browser-development-helpers.js", this);
   }
   Services.scriptloader.loadSubScript("chrome://browser/content/browser-pageActions.js", this);
-  Services.scriptloader.loadSubScript("chrome://browser/content/browser-sidebar.js", this);
+  Services.scriptloader.loadSubScript("chrome://browser/content/sidebar/browser-sidebar.js", this);
   Services.scriptloader.loadSubScript("chrome://browser/content/browser-tabsintitlebar.js", this);
   Services.scriptloader.loadSubScript("chrome://browser/content/browser-unified-extensions.js", this);
   Services.scriptloader.loadSubScript("chrome://browser/content/tabbrowser.js", this);
diff --git a/browser/base/content/contentTheme.js b/browser/base/content/contentTheme.js
index 3c46b80bec..2eb339d69b 100644
--- a/browser/base/content/contentTheme.js
+++ b/browser/base/content/contentTheme.js
@@ -43,8 +43,8 @@
           let browserStyle =
             element.ownerGlobal?.docShell?.chromeEventHandler.style;
 
+          element.toggleAttribute("lwt-newtab", !!rgbaChannels);
           if (!rgbaChannels) {
-            element.removeAttribute("lwt-newtab");
             element.toggleAttribute(
               "lwt-newtab-brighttext",
               prefersDarkQuery.matches
@@ -55,7 +55,6 @@
             return null;
           }
 
-          element.setAttribute("lwt-newtab", "true");
           const { r, g, b, a } = rgbaChannels;
           let darkMode = !_isTextColorDark(r, g, b);
           element.toggleAttribute("lwt-newtab-brighttext", darkMode);
@@ -159,15 +158,10 @@
      * @param {Object} event object containing the theme or query update.
      */
     handleEvent(event) {
-      const root = document.documentElement;
-
       if (event.type == "LightweightTheme:Set") {
-        let { data } = event.detail;
-        if (!data) {
-          data = {};
-        }
-        this._setProperties(root, data);
+        this._setProperties(event.detail.data || {});
       } else if (event.type == "change") {
+        const root = document.documentElement;
         // If a lightweight theme doesn't apply, update lwt-newtab-brighttext to
         // reflect prefers-color-scheme.
         if (!root.hasAttribute("lwt-newtab")) {
@@ -192,22 +186,23 @@
 
     /**
      * Apply theme data to an element
-     * @param {Element} root The element where the properties should be applied.
      * @param {Object} themeData The theme data.
      */
-    _setProperties(elem, themeData) {
+    _setProperties(themeData) {
+      const root = document.documentElement;
+      root.toggleAttribute("lwtheme", themeData.hasTheme);
       for (let [cssVarName, definition] of inContentVariableMap) {
         const { lwtProperty, processColor } = definition;
         let value = themeData[lwtProperty];
 
         if (processColor) {
-          value = processColor(value, elem);
+          value = processColor(value, root);
         } else if (value) {
           const { r, g, b, a } = value;
           value = `rgba(${r}, ${g}, ${b}, ${a})`;
         }
 
-        this._setProperty(elem, cssVarName, value);
+        this._setProperty(root, cssVarName, value);
       }
     },
   };
diff --git a/browser/base/content/main-popupset.inc.xhtml b/browser/base/content/main-popupset.inc.xhtml
index bff8d98b27..91b2483c53 100644
--- a/browser/base/content/main-popupset.inc.xhtml
+++ b/browser/base/content/main-popupset.inc.xhtml
@@ -100,7 +100,7 @@
               oncommand="FullScreen.setAutohide();"/>
     <menuitem contexttype="fullscreen"
               data-lazy-l10n-id="full-screen-exit"
-              oncommand="BrowserFullScreen();"/>
+              oncommand="BrowserCommands.fullScreen();"/>
   </menupopup>
 
   <!-- bug 415444/582485: event.stopPropagation is here for the cloned version
@@ -109,7 +109,7 @@
   -->
   <menupopup id="backForwardMenu"
              onpopupshowing="return FillHistoryMenu(event.target);"
-             oncommand="gotoHistoryIndex(event); event.stopPropagation();"/>
+             oncommand="BrowserCommands.gotoHistoryIndex(event); event.stopPropagation();"/>
   <tooltip id="aHTMLTooltip" page="true"/>
   <tooltip id="remoteBrowserTooltip"/>
 
@@ -262,6 +262,9 @@
               data-l10n-id="sidebar-menu-synced-tabs"
               class="sync-ui-item"
               oncommand="SidebarUI.show('viewTabsSidebar');"/>
+    <menuitem id="sidebar-switcher-megalist"
+              data-l10n-id="sidebar-menu-megalist"
+              oncommand="SidebarUI.show('viewMegalistSidebar');"/>
     <menuseparator/>
     <!-- Extension toolbarbuttons go here. -->
     <menuseparator id="sidebar-extensions-separator"/>
@@ -360,7 +363,7 @@
               oncommand="FullScreen.setAutohide();"/>
     <menuitem contexttype="fullscreen"
               data-lazy-l10n-id="full-screen-exit"
-              oncommand="BrowserFullScreen();"/>
+              oncommand="BrowserCommands.fullScreen();"/>
   </menupopup>
 
   <menupopup id="blockedPopupOptions"
@@ -412,7 +415,20 @@
     <hbox id="ctrlTab-showAll-container" pack="center"/>
   </panel>
 
-  <html:tab-preview id="tabbrowser-tab-preview" hidden="true" />
+  <!-- TODO: create lazily? -->
+  <panel id="tab-preview-panel"
+         type="arrow"
+         orient="vertical"
+         noautofocus="true"
+         norolluponanchor="true"
+         rolluponmousewheel="true"
+         consumeoutsideclicks="false">
+    <html:div class="tab-preview-text-container">
+      <html:div class="tab-preview-title"></html:div>
+      <html:div class="tab-preview-uri"></html:div>
+    </html:div>
+    <html:div class="tab-preview-thumbnail-container"></html:div>
+  </panel>
 
   <html:template id="pageActionPanelTemplate">
     <panel id="pageActionPanel"
@@ -624,7 +640,7 @@
               oncommand="gUnifiedExtensions.reportExtension(this.parentElement)" />
   </menupopup>
 
-  <menupopup id="translations-panel-settings-menupopup"
+  <menupopup id="full-page-translations-panel-settings-menupopup"
              onpopupshown="FullPageTranslationsPanel.handleSettingsPopupShownEvent()"
              onpopuphidden="FullPageTranslationsPanel.handleSettingsPopupHiddenEvent()">
     <menuitem class="always-offer-translations-menuitem"
diff --git a/browser/base/content/navigator-toolbox.inc.xhtml b/browser/base/content/navigator-toolbox.inc.xhtml
index fc19910726..a86d01adb2 100644
--- a/browser/base/content/navigator-toolbox.inc.xhtml
+++ b/browser/base/content/navigator-toolbox.inc.xhtml
@@ -491,13 +491,6 @@
                      tooltiptext="Ion"
                      onmousedown="switchToTabHavingURI('about:ion', true);"
                      onkeypress="switchToTabHavingURI('about:ion', true);"/>
-      <toolbarbutton id="whats-new-menu-button"
-                     class="toolbarbutton-1"
-                     delegatesanchor="true"
-                     hidden="true"
-                     badged="true"
-                     onmousedown="PanelUI.showSubView('PanelUI-whatsNew', this, event);"
-                     onkeypress="PanelUI.showSubView('PanelUI-whatsNew', this, event);"/>
       <toolbarbutton id="PanelUI-menu-button"
                      class="toolbarbutton-1"
                      delegatesanchor="true"
@@ -708,7 +701,7 @@
                    ondragenter="homeButtonObserver.onDragOver(event)"
                    ondrop="homeButtonObserver.onDrop(event)"
                    key="goHome"
-                   onclick="BrowserHome(event);"
+                   onclick="BrowserCommands.home(event);"
                    cui-areatype="toolbar"/>
 
     <toolbarbutton id="library-button" class="toolbarbutton-1 chromeclass-toolbar-additional subviewbutton-nav"
diff --git a/browser/base/content/nsContextMenu.js b/browser/base/content/nsContextMenu.js
index 031a32dddf..9769552dce 100644
--- a/browser/base/content/nsContextMenu.js
+++ b/browser/base/content/nsContextMenu.js
@@ -1619,7 +1619,7 @@ class nsContextMenu {
 
   // Open new "view source" window with the frame's URL.
   viewFrameSource() {
-    BrowserViewSourceOfDocument({
+    BrowserCommands.viewSourceOfDocument({
       browser: this.browser,
       URL: this.contentData.docLocation,
       outerWindowID: this.frameOuterWindowID,
@@ -1627,7 +1627,7 @@ class nsContextMenu {
   }
 
   viewInfo() {
-    BrowserPageInfo(
+    BrowserCommands.pageInfo(
       this.contentData.docLocation,
       null,
       null,
@@ -1637,7 +1637,7 @@ class nsContextMenu {
   }
 
   viewImageInfo() {
-    BrowserPageInfo(
+    BrowserCommands.pageInfo(
       this.contentData.docLocation,
       "mediaTab",
       this.imageInfo,
@@ -1661,7 +1661,7 @@ class nsContextMenu {
   }
 
   viewFrameInfo() {
-    BrowserPageInfo(
+    BrowserCommands.pageInfo(
       this.contentData.docLocation,
       null,
       null,
@@ -1982,7 +1982,7 @@ class nsContextMenu {
     // we give up waiting for the filename.
     function timerCallback() {}
     timerCallback.prototype = {
-      notify: function sLA_timer_notify(aTimer) {
+      notify: function sLA_timer_notify() {
         channel.cancel(NS_ERROR_SAVE_LINK_AS_TIMEOUT);
       },
     };
@@ -2493,23 +2493,6 @@ class nsContextMenu {
     openTrustedLinkIn(drmInfoURL, dest);
   }
 
-  /**
-   * Retrieves an instance of the TranslationsParent actor.
-   * @returns {TranslationsParent} - The TranslationsParent actor.
-   * @throws Throws if an instance of the actor cannot be retrieved.
-   */
-  static #getTranslationsActor() {
-    const actor =
-      gBrowser.selectedBrowser.browsingContext.currentWindowGlobal.getActor(
-        "Translations"
-      );
-
-    if (!actor) {
-      throw new Error("Unable to get the TranslationsParent");
-    }
-    return actor;
-  }
-
   /**
    * Determines if Full Page Translations is currently active on this page.
    *
@@ -2518,7 +2501,9 @@ class nsContextMenu {
   static #isFullPageTranslationsActive() {
     try {
       const { requestedTranslationPair } =
-        this.#getTranslationsActor().languageState;
+        TranslationsParent.getTranslationsActor(
+          gBrowser.selectedBrowser
+        ).languageState;
       return requestedTranslationPair !== null;
     } catch {
       // Failed to retrieve the Full Page Translations actor, do nothing.
@@ -2532,7 +2517,16 @@ class nsContextMenu {
    * @param {Event} event - The triggering event for opening the panel.
    */
   openSelectTranslationsPanel(event) {
-    SelectTranslationsPanel.open(event, this.#translationsLangPairPromise);
+    const context = this.contentData.context;
+    let screenX = context.screenXDevPx / window.devicePixelRatio;
+    let screenY = context.screenYDevPx / window.devicePixelRatio;
+    SelectTranslationsPanel.open(
+      event,
+      screenX,
+      screenY,
+      this.#getTextToTranslate(),
+      this.#translationsLangPairPromise
+    );
   }
 
   /**
@@ -2582,6 +2576,17 @@ class nsContextMenu {
     );
   }
 
+  /**
+   * Fetches text for translation, prioritizing selected text over link text.
+   *
+   * @returns {string} The text to translate.
+   */
+  #getTextToTranslate() {
+    return this.isTextSelected
+      ? this.selectionInfo.fullText.trim()
+      : this.linkTextStr.trim();
+  }
+
   /**
    * Displays or hides the translate-selection item in the context menu.
    */
@@ -2596,10 +2601,7 @@ class nsContextMenu {
       "browser.translations.select.enable"
     );
 
-    // Selected text takes precedence over link text.
-    const textToTranslate = this.isTextSelected
-      ? this.selectedText.trim()
-      : this.linkTextStr.trim();
+    const textToTranslate = this.#getTextToTranslate();
 
     translateSelectionItem.hidden =
       // Only show the item if the feature is enabled.
diff --git a/browser/base/content/pageinfo/pageInfo.js b/browser/base/content/pageinfo/pageInfo.js
index f3999a7cc5..c186e9572d 100644
--- a/browser/base/content/pageinfo/pageInfo.js
+++ b/browser/base/content/pageinfo/pageInfo.js
@@ -47,7 +47,7 @@ pageInfoTreeView.prototype = {
     return this.data[row][column.index] || "";
   },
 
-  setCellValue(row, column, value) {},
+  setCellValue() {},
 
   setCellText(row, column, value) {
     this.data[row][column.index] = value;
@@ -112,52 +112,52 @@ pageInfoTreeView.prototype = {
     this.sortcol = treecol.index;
   },
 
-  getRowProperties(row) {
+  getRowProperties() {
     return "";
   },
-  getCellProperties(row, column) {
+  getCellProperties() {
     return "";
   },
-  getColumnProperties(column) {
+  getColumnProperties() {
     return "";
   },
-  isContainer(index) {
+  isContainer() {
     return false;
   },
-  isContainerOpen(index) {
+  isContainerOpen() {
     return false;
   },
-  isSeparator(index) {
+  isSeparator() {
     return false;
   },
   isSorted() {
     return this.sortcol > -1;
   },
-  canDrop(index, orientation) {
+  canDrop() {
     return false;
   },
-  drop(row, orientation) {
+  drop() {
     return false;
   },
-  getParentIndex(index) {
+  getParentIndex() {
     return 0;
   },
-  hasNextSibling(index, after) {
+  hasNextSibling() {
     return false;
   },
-  getLevel(index) {
+  getLevel() {
     return 0;
   },
-  getImageSrc(row, column) {},
+  getImageSrc() {},
   getCellValue(row, column) {
     let col = column != null ? column : this.copycol;
     return row < 0 || col < 0 ? "" : this.data[row][col] || "";
   },
-  toggleOpenState(index) {},
-  cycleHeader(col) {},
+  toggleOpenState() {},
+  cycleHeader() {},
   selectionChanged() {},
-  cycleCell(row, column) {},
-  isEditable(row, column) {
+  cycleCell() {},
+  isEditable() {
     return false;
   },
 };
@@ -475,10 +475,10 @@ async function loadTab(args) {
 
 function openCacheEntry(key, cb) {
   var checkCacheListener = {
-    onCacheEntryCheck(entry) {
+    onCacheEntryCheck() {
       return Ci.nsICacheEntryOpenCallback.ENTRY_WANTED;
     },
-    onCacheEntryAvailable(entry, isNew, status) {
+    onCacheEntryAvailable(entry) {
       cb(entry);
     },
   };
@@ -1085,7 +1085,7 @@ let treeController = {
     return command == "cmd_copy" || command == "cmd_selectAll";
   },
 
-  isCommandEnabled(command) {
+  isCommandEnabled() {
     return true; // not worth checking for this
   },
 
diff --git a/browser/base/content/pageinfo/pageInfo.xhtml b/browser/base/content/pageinfo/pageInfo.xhtml
index baa017702f..cca293c534 100644
--- a/browser/base/content/pageinfo/pageInfo.xhtml
+++ b/browser/base/content/pageinfo/pageInfo.xhtml
@@ -10,6 +10,10 @@
   data-l10n-id="page-info-window"
   data-l10n-attrs="style"
   windowtype="Browser:page-info"
+#ifdef XP_MACOSX
+  drawtitle="true"
+  chromemargin="0,0,0,0"
+#endif
   onload="onLoadPageInfo()"
   align="stretch"
   screenX="10" screenY="10"
diff --git a/browser/base/content/pageinfo/permissions.js b/browser/base/content/pageinfo/permissions.js
index 7834e27c98..7803cbafe5 100644
--- a/browser/base/content/pageinfo/permissions.js
+++ b/browser/base/content/pageinfo/permissions.js
@@ -28,7 +28,7 @@ let gPermissions = SitePermissions.listPermissions()
   });
 
 var permissionObserver = {
-  observe(aSubject, aTopic, aData) {
+  observe(aSubject, aTopic) {
     if (aTopic == "perm-changed") {
       var permission = aSubject.QueryInterface(Ci.nsIPermission);
       if (
diff --git a/browser/base/content/pageinfo/security.js b/browser/base/content/pageinfo/security.js
index 3acd3cc452..9f9fb17bea 100644
--- a/browser/base/content/pageinfo/security.js
+++ b/browser/base/content/pageinfo/security.js
@@ -399,7 +399,7 @@ function realmHasPasswords(uri) {
  *
  * @param host - the domain name to look for in history
  */
-function previousVisitCount(host, endTimeReference) {
+function previousVisitCount(host) {
   if (!host) {
     return 0;
   }
diff --git a/browser/base/content/sanitizeDialog.js b/browser/base/content/sanitizeDialog.js
index 09a7d927df..1d9ea978b9 100644
--- a/browser/base/content/sanitizeDialog.js
+++ b/browser/base/content/sanitizeDialog.js
@@ -218,7 +218,7 @@ var gSanitizePromptDialog = {
     acceptButton.disabled = noneChecked;
   },
 
-  selectByTimespan() {
+  async selectByTimespan() {
     // This method is the onselect handler for the duration dropdown.  As a
     // result it's called a couple of times before onload calls init().
     if (!this._inited) {
@@ -247,7 +247,7 @@ var gSanitizePromptDialog = {
       }
       // make sure the sizes are updated in the new dialog
       else {
-        this.updateDataSizesInUI();
+        await this.updateDataSizesInUI();
       }
       return;
     }
@@ -267,7 +267,7 @@ var gSanitizePromptDialog = {
 
     if (!lazy.USE_OLD_DIALOG) {
       // We only update data sizes to display on the new dialog
-      this.updateDataSizesInUI();
+      await this.updateDataSizesInUI();
     }
   },
 
@@ -399,7 +399,7 @@ var gSanitizePromptDialog = {
     this.cacheSize = lazy.DownloadUtils.convertByteUnits(cacheSize);
 
     this._dataSizesUpdated = true;
-    this.updateDataSizesInUI();
+    await this.updateDataSizesInUI();
   },
 
   /**
@@ -473,7 +473,7 @@ var gSanitizePromptDialog = {
   /**
    * Updates data sizes displayed based on new selected timespan
    */
-  updateDataSizesInUI() {
+  async updateDataSizesInUI() {
     if (!this._dataSizesUpdated) {
       return;
     }
@@ -491,6 +491,7 @@ var gSanitizePromptDialog = {
     let timeSpanSelected = TIMESPAN_SELECTION_MAP[index];
     let [amount, unit] = this.siteDataSizes[timeSpanSelected];
 
+    document.l10n.pauseObserving();
     document.l10n.setAttributes(
       this._cookiesAndSiteDataCheckbox,
       "item-cookies-site-data-with-size",
@@ -503,6 +504,18 @@ var gSanitizePromptDialog = {
       "item-cached-content-with-size",
       { amount, unit }
     );
+
+    // make sure l10n updates are completed
+    await document.l10n.translateElements([
+      this._cookiesAndSiteDataCheckbox,
+      this._cacheCheckbox,
+    ]);
+
+    document.l10n.resumeObserving();
+
+    // the data sizes may have led to wrapping, resize dialog to make sure the buttons
+    // don't move out of view
+    await window.resizeDialog();
   },
 
   /**
diff --git a/browser/base/content/tabbrowser-tab.js b/browser/base/content/tabbrowser-tab.js
index ed3d4bb727..807a7d93fd 100644
--- a/browser/base/content/tabbrowser-tab.js
+++ b/browser/base/content/tabbrowser-tab.js
@@ -259,6 +259,14 @@
       return this._lastAccessed == Infinity ? Date.now() : this._lastAccessed;
     }
 
+    /**
+     * Returns a timestamp which attempts to represent the last time the user saw this tab.
+     * If the tab has not been active in this session, any lastAccessed is used. We
+     * differentiate between selected and explicitly visible; a selected tab in a hidden
+     * window is last seen when that window and tab were last visible.
+     * We use the application start time as a fallback value when no other suitable value
+     * is available.
+     */
     get lastSeenActive() {
       const isForegroundWindow =
         this.ownerGlobal ==
@@ -270,8 +278,16 @@
       if (this._lastSeenActive) {
         return this._lastSeenActive;
       }
-      // Use the application start time as the fallback value
-      return Services.startup.getStartupInfo().start.getTime();
+
+      const appStartTime = Services.startup.getStartupInfo().start.getTime();
+      if (!this._lastAccessed || this._lastAccessed >= appStartTime) {
+        // When the tab was created this session but hasn't been seen by the user,
+        // default to the application start time.
+        return appStartTime;
+      }
+      // The tab was restored from a previous session but never seen.
+      // Use the lastAccessed as the best proxy for when the user might have seen it.
+      return this._lastAccessed;
     }
 
     get _overPlayingIcon() {
@@ -457,7 +473,7 @@
       }
     }
 
-    on_mouseup(event) {
+    on_mouseup() {
       // Make sure that clear-selection is released.
       // Otherwise selection using Shift key may be broken.
       gBrowser.unlockClearMultiSelection();
@@ -706,11 +722,11 @@
       this.setAttribute("aria-describedby", "tabbrowser-tab-a11y-desc");
     }
 
-    on_focus(event) {
+    on_focus() {
       this.updateA11yDescription();
     }
 
-    on_AriaFocus(event) {
+    on_AriaFocus() {
       this.updateA11yDescription();
     }
   }
diff --git a/browser/base/content/tabbrowser-tabs.js b/browser/base/content/tabbrowser-tabs.js
index 36b6aeb390..7784e74ed3 100644
--- a/browser/base/content/tabbrowser-tabs.js
+++ b/browser/base/content/tabbrowser-tabs.js
@@ -34,6 +34,7 @@
       this.addEventListener("drop", this);
       this.addEventListener("dragend", this);
       this.addEventListener("dragleave", this);
+      this.addEventListener("mouseleave", this);
     }
 
     init() {
@@ -61,6 +62,7 @@
       this._hiddenSoundPlayingTabs = new Set();
       this._allTabs = null;
       this._visibleTabs = null;
+      this._previewPanel = null;
 
       var tab = this.allTabs[0];
       tab.label = this.emptyTabTitle;
@@ -125,12 +127,12 @@
 
       this.configureTooltip = () => {
         // fall back to original tooltip behavior if pref is not set
-        this.tooltip = this._showCardPreviews ? null : "tabbrowser-tab-tooltip";
-
-        // activate new tooltip behavior if pref is set
-        document
-          .getElementById("tabbrowser-tab-preview")
-          .toggleAttribute("hidden", !this._showCardPreviews);
+        if (this._showCardPreviews) {
+          this.tooltip = null;
+        } else {
+          this.tooltip = "tabbrowser-tab-tooltip";
+          this._previewPanel = null;
+        }
       };
       XPCOMUtils.defineLazyPreferenceGetter(
         this,
@@ -142,7 +144,7 @@
       this.configureTooltip();
     }
 
-    on_TabSelect(event) {
+    on_TabSelect() {
       this._handleTabSelect();
     }
 
@@ -188,23 +190,23 @@
     }
 
     on_TabHoverStart(event) {
-      if (this._showCardPreviews) {
-        const previewContainer = document.getElementById(
-          "tabbrowser-tab-preview"
+      if (!this._showCardPreviews) {
+        return;
+      }
+      if (!this._previewPanel) {
+        // load the tab preview component
+        const TabPreviewPanel = ChromeUtils.importESModule(
+          "chrome://browser/content/tabpreview/tab-preview-panel.mjs"
+        ).default;
+        this._previewPanel = new TabPreviewPanel(
+          document.getElementById("tab-preview-panel")
         );
-        previewContainer.tab = event.target;
       }
+      this._previewPanel.activate(event.target);
     }
 
     on_TabHoverEnd(event) {
-      if (this._showCardPreviews) {
-        const previewContainer = document.getElementById(
-          "tabbrowser-tab-preview"
-        );
-        if (previewContainer.tab === event.target) {
-          previewContainer.tab = null;
-        }
-      }
+      this._previewPanel?.deactivate(event.target);
     }
 
     on_transitionend(event) {
@@ -241,7 +243,7 @@
       }
 
       if (!this._blockDblClick) {
-        BrowserOpenTab();
+        BrowserCommands.openTab();
       }
 
       event.preventDefault();
@@ -333,7 +335,7 @@
             (!RTL_UI && event.clientX > endOfTab) ||
             (RTL_UI && event.clientX < endOfTab)
           ) {
-            BrowserOpenTab();
+            BrowserCommands.openTab();
           }
         } else {
           return;
@@ -450,6 +452,7 @@
         return;
       }
 
+      this._previewPanel?.deactivate();
       this.startTabDrag(event, tab);
     }
 
@@ -1092,6 +1095,10 @@
       return children;
     }
 
+    get previewPanel() {
+      return this._previewPanel;
+    }
+
     _getVisibleTabs() {
       if (!this._visibleTabs) {
         this._visibleTabs = Array.prototype.filter.call(
@@ -1205,7 +1212,7 @@
       };
     }
 
-    observe(aSubject, aTopic, aData) {
+    observe(aSubject, aTopic) {
       switch (aTopic) {
         case "nsPref:changed":
           // This is has to deal with changes in
@@ -1851,6 +1858,9 @@
             this._unlockTabSizing();
           }
           break;
+        case "mouseleave":
+          this._previewPanel?.deactivate();
+          break;
         default:
           let methodName = `on_${aEvent.type}`;
           if (methodName in this) {
diff --git a/browser/base/content/tabbrowser.js b/browser/base/content/tabbrowser.js
index 54a801939a..3bca0b6d30 100644
--- a/browser/base/content/tabbrowser.js
+++ b/browser/base/content/tabbrowser.js
@@ -893,14 +893,6 @@
         : "";
     },
 
-    getTabModalPromptBox(aBrowser) {
-      let browser = aBrowser || this.selectedBrowser;
-      if (!browser.tabModalPromptBox) {
-        browser.tabModalPromptBox = new TabModalPromptBox(browser);
-      }
-      return browser.tabModalPromptBox;
-    },
-
     getTabDialogBox(aBrowser) {
       if (!aBrowser) {
         throw new Error("aBrowser is required");
@@ -1320,31 +1312,6 @@
           this.addToMultiSelectedTabs(oldTab);
         }
 
-        if (oldBrowser != newBrowser && oldBrowser.getInPermitUnload) {
-          oldBrowser.getInPermitUnload(inPermitUnload => {
-            if (!inPermitUnload) {
-              return;
-            }
-            // Since the user is switching away from a tab that has
-            // a beforeunload prompt active, we remove the prompt.
-            // This prevents confusing user flows like the following:
-            //   1. User attempts to close Firefox
-            //   2. User switches tabs (ingoring a beforeunload prompt)
-            //   3. User returns to tab, presses "Leave page"
-            let promptBox = this.getTabModalPromptBox(oldBrowser);
-            let prompts = promptBox.listPrompts();
-            // There might not be any prompts here if the tab was closed
-            // while in an onbeforeunload prompt, which will have
-            // destroyed aforementioned prompt already, so check there's
-            // something to remove, first:
-            if (prompts.length) {
-              // NB: This code assumes that the beforeunload prompt
-              //     is the top-most prompt on the tab.
-              prompts[prompts.length - 1].abortPrompt();
-            }
-          });
-        }
-
         if (!gMultiProcessBrowser) {
           this._adjustFocusBeforeTabSwitch(oldTab, newTab);
           this._adjustFocusAfterTabSwitch(newTab);
@@ -1439,19 +1406,6 @@
         newBrowser.tabDialogBox.focus();
         return;
       }
-      if (newBrowser.hasAttribute("tabmodalPromptShowing")) {
-        // If there's a tabmodal prompt showing, focus it.
-        let prompts = newBrowser.tabModalPromptBox.listPrompts();
-        let prompt = prompts[prompts.length - 1];
-        // @tabmodalPromptShowing is also set for other tab modal prompts
-        // (e.g. the Payment Request dialog) so there may not be a <tabmodalprompt>.
-        // Bug 1492814 will implement this for the Payment Request dialog.
-        if (prompt) {
-          prompt.Dialog.setDefaultFocus();
-          return;
-        }
-      }
-
       // Focus the location bar if it was previously focused for that tab.
       // In full screen mode, only bother making the location bar visible
       // if the tab is a blank one.
@@ -2097,18 +2051,6 @@
       // doesn't keep the window alive.
       b.permanentKey = new (Cu.getGlobalForObject(Services).Object)();
 
-      // Ensure that SessionStore has flushed any session history state from the
-      // content process before we this browser's remoteness.
-      if (!Services.appinfo.sessionHistoryInParent) {
-        b.prepareToChangeRemoteness = () =>
-          SessionStore.prepareToChangeRemoteness(b);
-        b.afterChangeRemoteness = switchId => {
-          let tab = this.getTabForBrowser(b);
-          SessionStore.finishTabRemotenessChange(tab, switchId);
-          return true;
-        };
-      }
-
       const defaultBrowserAttributes = {
         contextmenu: "contentAreaContextMenu",
         message: "true",
@@ -2682,8 +2624,6 @@
           animate,
           userContextId,
           openerTab,
-          createLazyBrowser,
-          skipAnimation,
           pinned,
           noInitialLabel,
           skipBackgroundNotify,
@@ -2849,8 +2789,6 @@
       uriString,
       userContextId,
       openerTab,
-      createLazyBrowser,
-      skipAnimation,
       pinned,
       noInitialLabel,
       skipBackgroundNotify,
@@ -3367,7 +3305,7 @@
       Services.telemetry.setEventRecordingEnabled("close_tab_warning", true);
       let closeTabEnumKey =
         Object.entries(this.closingTabsEnum)
-          .find(([k, v]) => v == aCloseTabs)?.[0]
+          .find(([, v]) => v == aCloseTabs)?.[0]
           ?.toLowerCase() || "some";
 
       let warnCheckbox = warnOnClose.value ? "checked" : "unchecked";
@@ -3674,6 +3612,8 @@
       tabs,
       {
         animate,
+        // See bug 1883051
+        // eslint-disable-next-line no-unused-vars
         suppressWarnAboutClosingWindow,
         skipPermitUnload,
         skipRemoves,
@@ -4406,7 +4346,7 @@
       } // Do nothing
 
       if (event.button == 1) {
-        BrowserOpenTab({ event });
+        BrowserCommands.openTab({ event });
         // Stop the propagation of the click event, to prevent the event from being
         // handled more than once.
         // E.g. see https://bugzilla.mozilla.org/show_bug.cgi?id=1657992#c4
@@ -5849,7 +5789,7 @@
       }
     },
 
-    observe(aSubject, aTopic, aData) {
+    observe(aSubject, aTopic) {
       switch (aTopic) {
         case "contextual-identity-updated": {
           let identity = aSubject.wrappedJSObject;
@@ -6105,12 +6045,7 @@
               );
               if (permission != Services.perms.ALLOW_ACTION) {
                 // Tell the prompt box we want to show the user a checkbox:
-                let tabPrompt = Services.prefs.getBoolPref(
-                  "prompts.contentPromptSubDialog"
-                )
-                  ? this.getTabDialogBox(tabForEvent.linkedBrowser)
-                  : this.getTabModalPromptBox(tabForEvent.linkedBrowser);
-
+                let tabPrompt = this.getTabDialogBox(tabForEvent.linkedBrowser);
                 tabPrompt.onNextPromptShowAllowFocusCheckboxFor(
                   promptPrincipal
                 );
@@ -6352,7 +6287,7 @@
         let oldUserTypedValue = browser.userTypedValue;
         let hadStartedLoad = browser.didStartLoadSinceLastUserTyping();
 
-        let didChange = didChangeEvent => {
+        let didChange = () => {
           browser.userTypedValue = oldUserTypedValue;
           if (hadStartedLoad) {
             browser.urlbarChangeTracker.startedLoad();
@@ -7461,7 +7396,7 @@ var TabBarVisibility = {
 
     toolbar.collapsed = collapse;
     let navbar = document.getElementById("nav-bar");
-    navbar.setAttribute("tabs-hidden", collapse);
+    navbar.toggleAttribute("tabs-hidden", collapse);
 
     document.getElementById("menu_closeWindow").hidden = collapse;
     document.l10n.setAttributes(
@@ -7785,7 +7720,7 @@ var TabContextMenu = {
     }
   },
 
-  closeContextTabs(event) {
+  closeContextTabs() {
     if (this.contextTab.multiselected) {
       gBrowser.removeMultiSelectedTabs();
     } else {
diff --git a/browser/base/content/test/about/browser_aboutCertError.js b/browser/base/content/test/about/browser_aboutCertError.js
index 9af82b807f..5939b026bd 100644
--- a/browser/base/content/test/about/browser_aboutCertError.js
+++ b/browser/base/content/test/about/browser_aboutCertError.js
@@ -121,7 +121,7 @@ add_task(async function checkReturnToPreviousPage() {
       "pageshow",
       true
     );
-    await SpecialPowers.spawn(bc, [useFrame], async function (subFrame) {
+    await SpecialPowers.spawn(bc, [useFrame], async function () {
       let returnButton = content.document.getElementById("returnButton");
       returnButton.click();
     });
@@ -544,7 +544,7 @@ add_task(async function checkViewSource() {
   certOverrideService.clearValidityOverride("expired.example.com", -1, {});
 
   loaded = BrowserTestUtils.waitForErrorPage(browser);
-  BrowserReloadSkipCache();
+  BrowserCommands.reloadSkipCache();
   await loaded;
 
   BrowserTestUtils.removeTab(gBrowser.selectedTab);
diff --git a/browser/base/content/test/alerts/browser_notification_open_settings.js b/browser/base/content/test/alerts/browser_notification_open_settings.js
index ed51cd782b..e7f1c28251 100644
--- a/browser/base/content/test/alerts/browser_notification_open_settings.js
+++ b/browser/base/content/test/alerts/browser_notification_open_settings.js
@@ -14,7 +14,7 @@ add_task(async function test_settingsOpen_observer() {
       gBrowser,
       url: "about:robots",
     },
-    async function dummyTabTask(aBrowser) {
+    async function dummyTabTask() {
       // Ensure preferences is loaded before removing the tab.
       let syncPaneLoadedPromise = TestUtils.topicObserved(
         "sync-pane-loaded",
diff --git a/browser/base/content/test/alerts/head.js b/browser/base/content/test/alerts/head.js
index 4be18f6c41..eaf3a2bb74 100644
--- a/browser/base/content/test/alerts/head.js
+++ b/browser/base/content/test/alerts/head.js
@@ -20,7 +20,7 @@ async function addNotificationPermission(originString) {
  */
 function promiseWindowClosed(window) {
   return new Promise(function (resolve) {
-    Services.ww.registerNotification(function observer(subject, topic, data) {
+    Services.ww.registerNotification(function observer(subject, topic) {
       if (topic == "domwindowclosed" && subject == window) {
         Services.ww.unregisterNotification(observer);
         resolve();
diff --git a/browser/base/content/test/captivePortal/browser_captivePortal_certErrorUI.js b/browser/base/content/test/captivePortal/browser_captivePortal_certErrorUI.js
index 6389338a6f..b65a419884 100644
--- a/browser/base/content/test/captivePortal/browser_captivePortal_certErrorUI.js
+++ b/browser/base/content/test/captivePortal/browser_captivePortal_certErrorUI.js
@@ -117,7 +117,7 @@ add_task(async function testCaptivePortalAdvancedPanel() {
     await BrowserTestUtils.waitForLocationChange(gBrowser, BAD_CERT_PAGE);
     info("(waitForLocationChange resolved)");
   })();
-  await SpecialPowers.spawn(browser, [BAD_CERT_PAGE], async expectedURL => {
+  await SpecialPowers.spawn(browser, [BAD_CERT_PAGE], async () => {
     const doc = content.document;
     let advancedButton = doc.getElementById("advancedButton");
     await ContentTaskUtils.waitForCondition(
diff --git a/browser/base/content/test/contextMenu/browser.toml b/browser/base/content/test/contextMenu/browser.toml
index 3eb6a1d606..660f6a955b 100644
--- a/browser/base/content/test/contextMenu/browser.toml
+++ b/browser/base/content/test/contextMenu/browser.toml
@@ -8,7 +8,6 @@ support-files = [
   "subtst_contextmenu_xul.xhtml",
   "ctxmenu-image.png",
   "../general/head.js",
-  "../general/video.ogg",
   "../general/audio.ogg",
   "../../../../../toolkit/components/pdfjs/test/file_pdfjs_test.pdf",
   "contextmenu_common.js",
@@ -19,6 +18,7 @@ support-files = [
 ["browser_bug1798178.js"]
 
 ["browser_contextmenu.js"]
+support-files = [ "../general/video.webm"  ]
 tags = "fullscreen"
 skip-if = [
   "os == 'linux'",
@@ -86,6 +86,8 @@ skip-if = ["os == 'linux' && socketprocess_networking"]
 
 ["browser_strip_on_share_link.js"]
 
+["browser_strip_on_share_nested_link.js"]
+
 ["browser_utilityOverlay.js"]
 https_first_disabled = true
 skip-if = ["os == 'linux' && socketprocess_networking"]
diff --git a/browser/base/content/test/contextMenu/browser_contextmenu.js b/browser/base/content/test/contextMenu/browser_contextmenu.js
index ebeb4bdb04..00da3113c6 100644
--- a/browser/base/content/test/contextMenu/browser_contextmenu.js
+++ b/browser/base/content/test/contextMenu/browser_contextmenu.js
@@ -137,7 +137,7 @@ add_task(async function test_setup_html() {
     audio.loop = true;
     audio.src = "audio.ogg";
     video.loop = true;
-    video.src = "video.ogg";
+    video.src = "video.webm";
 
     let awaitPause = ContentTaskUtils.waitForEvent(audio, "pause");
     await ContentTaskUtils.waitForCondition(
diff --git a/browser/base/content/test/contextMenu/browser_contextmenu_badiframe.js b/browser/base/content/test/contextMenu/browser_contextmenu_badiframe.js
index 991a55af70..57d9808f5d 100644
--- a/browser/base/content/test/contextMenu/browser_contextmenu_badiframe.js
+++ b/browser/base/content/test/contextMenu/browser_contextmenu_badiframe.js
@@ -30,7 +30,7 @@ async function openTestPage() {
   let pageAndIframesLoaded = BrowserTestUtils.browserLoaded(
     browser,
     true /* includeSubFrames */,
-    url => {
+    () => {
       expectedLoads--;
       return !expectedLoads;
     },
diff --git a/browser/base/content/test/contextMenu/browser_contextmenu_save_blocked.js b/browser/base/content/test/contextMenu/browser_contextmenu_save_blocked.js
index 062fbeac08..7e6b71e8e4 100644
--- a/browser/base/content/test/contextMenu/browser_contextmenu_save_blocked.js
+++ b/browser/base/content/test/contextMenu/browser_contextmenu_save_blocked.js
@@ -64,7 +64,7 @@ add_task(async function test_save_link_blocked_by_extension() {
           setTimeout(resolve, 0);
         };
 
-        MockFilePicker.showCallback = function (fp) {
+        MockFilePicker.showCallback = function () {
           ok(false, "filepicker should never been shown");
           setTimeout(resolve, 0);
           return Ci.nsIFilePicker.returnCancel;
diff --git a/browser/base/content/test/contextMenu/browser_strip_on_share_nested_link.js b/browser/base/content/test/contextMenu/browser_strip_on_share_nested_link.js
new file mode 100644
index 0000000000..d11649e648
--- /dev/null
+++ b/browser/base/content/test/contextMenu/browser_strip_on_share_nested_link.js
@@ -0,0 +1,162 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+let listService;
+
+const TEST_URL =
+  "https://example.com/browser/browser/base/content/test/general/dummy_page.html";
+
+add_setup(async function () {
+  await SpecialPowers.pushPrefEnv({
+    set: [["privacy.query_stripping.strip_list", "stripParam"]],
+  });
+
+  // Get the list service so we can wait for it to be fully initialized before running tests.
+  listService = Cc["@mozilla.org/query-stripping-list-service;1"].getService(
+    Ci.nsIURLQueryStrippingListService
+  );
+
+  await listService.testWaitForInit();
+});
+
+/*
+Tests the strip-on-share feature for in-content links with nested urls
+*/
+
+// Testing nested stripping with global params
+add_task(async function testNestedStrippingGlobalParam() {
+  let validUrl =
+    "https://www.example.com/?test=https%3A%2F%2Fwww.example.net%2F%3Futm_ad%3D1234";
+  let shortenedUrl =
+    "https://www.example.com/?test=https%3A%2F%2Fwww.example.net%2F";
+  await testStripOnShare({
+    originalURI: validUrl,
+    strippedURI: shortenedUrl,
+  });
+});
+
+// Testing nested stripping with site specific params
+add_task(async function testNestedStrippingSiteSpecific() {
+  let validUrl =
+    "https://www.example.com/?test=https%3A%2F%2Fwww.example.net%2F%3Ftest_3%3D1234";
+  let shortenedUrl =
+    "https://www.example.com/?test=https%3A%2F%2Fwww.example.net%2F";
+  await testStripOnShare({
+    originalURI: validUrl,
+    strippedURI: shortenedUrl,
+  });
+});
+
+// Testing nested stripping with incorrect site specific params
+add_task(async function testNoStrippedNestedParam() {
+  let validUrl =
+    "https://www.example.com/?test=https%3A%2F%2Fwww.example.com%2F%3Ftest_3%3D1234";
+  let shortenedUrl =
+    "https://www.example.com/?test=https%3A%2F%2Fwww.example.com%2F%3Ftest_3%3D1234";
+  await testStripOnShare({
+    originalURI: validUrl,
+    strippedURI: shortenedUrl,
+  });
+});
+
+// Testing order of stripping for nested stripping
+add_task(async function testOrderOfStripping() {
+  let validUrl =
+    "https://www.example.com/?test_1=https%3A%2F%2Fwww.example.net%2F%3Ftest_3%3D1234";
+  let shortenedUrl = "https://www.example.com/";
+  await testStripOnShare({
+    originalURI: validUrl,
+    strippedURI: shortenedUrl,
+  });
+});
+
+// Testing correct scoping of site specific params for nested stripping
+add_task(async function testMultipleQueryParamsWithNestedStripping() {
+  let validUrl =
+    "https://www.example.com/?test_3=1234&test=https%3A%2F%2Fwww.example.net%2F%3Ftest_3%3D1234";
+  let shortenedUrl =
+    "https://www.example.com/?test_3=1234&test=https%3A%2F%2Fwww.example.net%2F";
+  await testStripOnShare({
+    originalURI: validUrl,
+    strippedURI: shortenedUrl,
+  });
+});
+
+// Testing functionality with no https pages
+add_task(async function testNonHTTPsPages() {
+  let validUrl = "https://www.example.com/?test_2=1234&test=about%3A%3Aconfig";
+  let shortenedUrl = "https://www.example.com/?test=about%3A%3Aconfig";
+  await testStripOnShare({
+    originalURI: validUrl,
+    strippedURI: shortenedUrl,
+  });
+});
+
+/**
+ * Opens a new tab, opens the context menu and checks that the strip-on-share menu item is visible.
+ * Checks that the stripped version of the url is copied to the clipboard.
+ *
+ * @param {string} originalURI - The orginal url before the stripping occurs
+ * @param {string} strippedURI - The expected url after stripping occurs
+ */
+async function testStripOnShare({ originalURI, strippedURI }) {
+  await SpecialPowers.pushPrefEnv({
+    set: [
+      ["privacy.query_stripping.strip_on_share.enabled", true],
+      ["privacy.query_stripping.strip_on_share.enableTestMode", true],
+    ],
+  });
+
+  let testJson = {
+    global: {
+      queryParams: ["utm_ad"],
+      topLevelSites: ["*"],
+    },
+    example: {
+      queryParams: ["test_2", "test_1"],
+      topLevelSites: ["www.example.com"],
+    },
+    exampleNet: {
+      queryParams: ["test_3", "test_4"],
+      topLevelSites: ["www.example.net"],
+    },
+  };
+
+  await listService.testSetList(testJson);
+
+  await BrowserTestUtils.withNewTab(TEST_URL, async function (browser) {
+    // Prepare a link
+    await SpecialPowers.spawn(browser, [originalURI], function (startingURI) {
+      let link = content.document.createElement("a");
+      link.href = startingURI;
+      link.textContent = "link with query param";
+      link.id = "link";
+      content.document.body.appendChild(link);
+    });
+    let contextMenu = document.getElementById("contentAreaContextMenu");
+    // Open the context menu
+    let awaitPopupShown = BrowserTestUtils.waitForEvent(
+      contextMenu,
+      "popupshown"
+    );
+    await BrowserTestUtils.synthesizeMouseAtCenter(
+      "#link",
+      { type: "contextmenu", button: 2 },
+      browser
+    );
+    await awaitPopupShown;
+    let awaitPopupHidden = BrowserTestUtils.waitForEvent(
+      contextMenu,
+      "popuphidden"
+    );
+    let stripOnShare = contextMenu.querySelector("#context-stripOnShareLink");
+    Assert.ok(BrowserTestUtils.isVisible(stripOnShare), "Menu item is visible");
+    // Make sure the stripped link will be copied to the clipboard
+    await SimpleTest.promiseClipboardChange(strippedURI, () => {
+      contextMenu.activateItem(stripOnShare);
+    });
+    await awaitPopupHidden;
+  });
+}
diff --git a/browser/base/content/test/contextMenu/contextmenu_common.js b/browser/base/content/test/contextMenu/contextmenu_common.js
index ac61aa2a3a..2c9a1967f6 100644
--- a/browser/base/content/test/contextMenu/contextmenu_common.js
+++ b/browser/base/content/test/contextMenu/contextmenu_common.js
@@ -39,7 +39,7 @@ function closeContextMenu() {
   contextMenu.hidePopup();
 }
 
-function getVisibleMenuItems(aMenu, aData) {
+function getVisibleMenuItems(aMenu) {
   var items = [];
   var accessKeys = {};
   for (var i = 0; i < aMenu.children.length; i++) {
@@ -65,7 +65,7 @@ function getVisibleMenuItems(aMenu, aData) {
       var label = item.getAttribute("label");
       ok(label.length, "menuitem " + item.id + " has a label");
       if (isGenerated) {
-        is(key, "", "Generated items shouldn't have an access key");
+        is(key, null, "Generated items shouldn't have an access key");
         items.push("*" + label);
       } else if (
         item.id.indexOf("spell-check-dictionary-") != 0 &&
diff --git a/browser/base/content/test/contextMenu/subtst_contextmenu.html b/browser/base/content/test/contextMenu/subtst_contextmenu.html
index 2c263fbce4..2facd9fecc 100644
--- a/browser/base/content/test/contextMenu/subtst_contextmenu.html
+++ b/browser/base/content/test/contextMenu/subtst_contextmenu.html
@@ -26,14 +26,14 @@ document.getElementById("shadow-host-in-link").attachShadow({ mode: "closed" }).
   <image id="test-svg-image" href="ctxmenu-image.png"/>
 </svg>
 <canvas id="test-canvas" width="100" height="100" style="background-color: blue"></canvas>
-<video controls id="test-video-ok"  src="video.ogg" width="100" height="100" style="background-color: green"></video>
+<video controls id="test-video-ok"  src="video.webm" width="100" height="100" style="background-color: green"></video>
 <video id="test-audio-in-video" src="audio.ogg" width="100" height="100" style="background-color: red"></video>
 <video controls id="test-video-bad" src="bogus.duh" width="100" height="100" style="background-color: orange"></video>
 <video controls id="test-video-bad2" width="100" height="100" style="background-color: yellow">
   <source src="bogus.duh" type="video/durrrr;">
 </video>
 <iframe id="test-iframe" width="98"  height="98" style="border: 1px solid black"></iframe>
-<iframe id="test-video-in-iframe" src="video.ogg" width="98" height="98" style="border: 1px solid black"></iframe>
+<iframe id="test-video-in-iframe" src="video.webm" width="98" height="98" style="border: 1px solid black"></iframe>
 <iframe id="test-audio-in-iframe" src="audio.ogg" width="98" height="98" style="border: 1px solid black"></iframe>
 <iframe id="test-image-in-iframe" src="ctxmenu-image.png" width="98" height="98" style="border: 1px solid black"></iframe>
 <iframe id="test-pdf-viewer-in-frame" src="file_pdfjs_test.pdf" width="100" height="100" style="border: 1px solid black"></iframe>
diff --git a/browser/base/content/test/contextMenu/subtst_contextmenu_webext.html b/browser/base/content/test/contextMenu/subtst_contextmenu_webext.html
index ac3b5415dd..be45c2ddd0 100644
--- a/browser/base/content/test/contextMenu/subtst_contextmenu_webext.html
+++ b/browser/base/content/test/contextMenu/subtst_contextmenu_webext.html
@@ -7,6 +7,6 @@
 <body>
   Browser context menu subtest.
   <a href="moz-extension://foo-bar/tab.html" id="link">Link to an extension resource</a>
-  <video src="moz-extension://foo-bar/video.ogg" id="video"></video>
+  <video src="moz-extension://foo-bar/video.webm" id="video"></video>
 </body>
 </html>
diff --git a/browser/base/content/test/favicons/browser_favicon_change_not_in_document.js b/browser/base/content/test/favicons/browser_favicon_change_not_in_document.js
index b8215dcc3e..85240aaa95 100644
--- a/browser/base/content/test/favicons/browser_favicon_change_not_in_document.js
+++ b/browser/base/content/test/favicons/browser_favicon_change_not_in_document.js
@@ -36,8 +36,8 @@ add_task(async function () {
   ));
   let domLinkAddedFired = 0;
   let domLinkChangedFired = 0;
-  const linkAddedHandler = event => domLinkAddedFired++;
-  const linkChangedhandler = event => domLinkChangedFired++;
+  const linkAddedHandler = () => domLinkAddedFired++;
+  const linkChangedhandler = () => domLinkChangedFired++;
   BrowserTestUtils.addContentEventListener(
     gBrowser.selectedBrowser,
     "DOMLinkAdded",
@@ -80,8 +80,8 @@ add_task(async function () {
 
   let domLinkAddedFired = 0;
   let domLinkChangedFired = 0;
-  const linkAddedHandler = event => domLinkAddedFired++;
-  const linkChangedhandler = event => domLinkChangedFired++;
+  const linkAddedHandler = () => domLinkAddedFired++;
+  const linkChangedhandler = () => domLinkChangedFired++;
   BrowserTestUtils.addContentEventListener(
     browser,
     "DOMLinkAdded",
diff --git a/browser/base/content/test/favicons/browser_favicon_load.js b/browser/base/content/test/favicons/browser_favicon_load.js
index 10c2b8f24e..7b78ae494f 100644
--- a/browser/base/content/test/favicons/browser_favicon_load.js
+++ b/browser/base/content/test/favicons/browser_favicon_load.js
@@ -50,7 +50,7 @@ function FaviconObserver(aPageURI, aFaviconURL, aTailingEnabled) {
 }
 
 FaviconObserver.prototype = {
-  observe(aSubject, aTopic, aData) {
+  observe(aSubject, aTopic) {
     // Make sure that the topic is 'http-on-modify-request'.
     if (aTopic === "http-on-modify-request") {
       let httpChannel = aSubject.QueryInterface(Ci.nsIHttpChannel);
diff --git a/browser/base/content/test/favicons/browser_favicon_nostore.js b/browser/base/content/test/favicons/browser_favicon_nostore.js
index 3fec666bbe..c12c7a87cd 100644
--- a/browser/base/content/test/favicons/browser_favicon_nostore.js
+++ b/browser/base/content/test/favicons/browser_favicon_nostore.js
@@ -140,20 +140,17 @@ add_task(async function root_icon_stored() {
     response.write("<html>A page without icon</html>");
   });
 
-  let noStorePromise = TestUtils.topicObserved(
-    "http-on-stop-request",
-    (s, t, d) => {
-      let chan = s.QueryInterface(Ci.nsIHttpChannel);
-      return chan?.URI.spec == "http://www.nostore.com/favicon.ico";
-    }
-  ).then(([chan]) => chan.isNoStoreResponse());
+  let noStorePromise = TestUtils.topicObserved("http-on-stop-request", s => {
+    let chan = s.QueryInterface(Ci.nsIHttpChannel);
+    return chan?.URI.spec == "http://www.nostore.com/favicon.ico";
+  }).then(([chan]) => chan.isNoStoreResponse());
 
   await BrowserTestUtils.withNewTab(
     {
       gBrowser,
       url: "http://www.nostore.com/page",
     },
-    async function (browser) {
+    async function () {
       await TestUtils.waitForCondition(async () => {
         let uri = await new Promise(resolve =>
           PlacesUtils.favicons.getFaviconURLForPage(
diff --git a/browser/base/content/test/favicons/browser_favicon_referer.js b/browser/base/content/test/favicons/browser_favicon_referer.js
index ed332e7413..9fee9771b0 100644
--- a/browser/base/content/test/favicons/browser_favicon_referer.js
+++ b/browser/base/content/test/favicons/browser_favicon_referer.js
@@ -14,7 +14,7 @@ add_task(async function test_check_referrer_for_discovered_favicon() {
     async browser => {
       let referrerPromise = TestUtils.topicObserved(
         "http-on-modify-request",
-        (s, t, d) => {
+        s => {
           let chan = s.QueryInterface(Ci.nsIHttpChannel);
           return chan.URI.spec == "http://mochi.test:8888/favicon.ico";
         }
@@ -42,7 +42,7 @@ add_task(
       async browser => {
         let referrerPromise = TestUtils.topicObserved(
           "http-on-modify-request",
-          (s, t, d) => {
+          s => {
             let chan = s.QueryInterface(Ci.nsIHttpChannel);
             return chan.URI.spec == `${FOLDER}file_favicon.png`;
           }
diff --git a/browser/base/content/test/favicons/browser_missing_favicon.js b/browser/base/content/test/favicons/browser_missing_favicon.js
index f619425909..fd60d362b4 100644
--- a/browser/base/content/test/favicons/browser_missing_favicon.js
+++ b/browser/base/content/test/favicons/browser_missing_favicon.js
@@ -28,7 +28,7 @@ add_task(async () => {
       is(browser.mIconURL, null, "Should have blanked the icon.");
       is(
         gBrowser.getTabForBrowser(browser).getAttribute("image"),
-        "",
+        null,
         "Should have blanked the tab icon."
       );
     }
diff --git a/browser/base/content/test/forms/browser_selectpopup.js b/browser/base/content/test/forms/browser_selectpopup.js
index abcdee486f..72112974c2 100644
--- a/browser/base/content/test/forms/browser_selectpopup.js
+++ b/browser/base/content/test/forms/browser_selectpopup.js
@@ -186,7 +186,7 @@ async function doSelectTests(contentType, content) {
   );
 
   // Backspace should not go back
-  let handleKeyPress = function (event) {
+  let handleKeyPress = function () {
     ok(false, "Should not get keypress event");
   };
   window.addEventListener("keypress", handleKeyPress);
@@ -708,7 +708,7 @@ add_task(async function test_mousemove_correcttarget() {
       window,
       "sizemodechange"
     );
-    BrowserFullScreen();
+    BrowserCommands.fullScreen();
     await sizeModeChanged;
     await popupHiddenPromise;
   }
diff --git a/browser/base/content/test/forms/browser_selectpopup_colors.js b/browser/base/content/test/forms/browser_selectpopup_colors.js
index 63cece0ce5..f4b3e8a516 100644
--- a/browser/base/content/test/forms/browser_selectpopup_colors.js
+++ b/browser/base/content/test/forms/browser_selectpopup_colors.js
@@ -255,7 +255,7 @@ function rgbaToString(parsedColor) {
   return `rgba(${r}, ${g}, ${b}, ${a})`;
 }
 
-function testOptionColors(test, index, item, menulist) {
+function testOptionColors(test, index, item) {
   // The label contains a JSON string of the expected colors for
   // `color` and `background-color`.
   let expected = JSON.parse(item.label);
diff --git a/browser/base/content/test/forms/browser_selectpopup_dir.js b/browser/base/content/test/forms/browser_selectpopup_dir.js
index aaf4a61fc2..a0ad90d909 100644
--- a/browser/base/content/test/forms/browser_selectpopup_dir.js
+++ b/browser/base/content/test/forms/browser_selectpopup_dir.js
@@ -13,7 +13,7 @@ add_task(async function () {
       gBrowser,
       url,
     },
-    async function (browser) {
+    async function () {
       let popup = await openSelectPopup("click");
       is(popup.style.direction, "rtl", "Should be the right dir");
     }
diff --git a/browser/base/content/test/forms/browser_selectpopup_large.js b/browser/base/content/test/forms/browser_selectpopup_large.js
index 722e0d9588..40f6d1b160 100644
--- a/browser/base/content/test/forms/browser_selectpopup_large.js
+++ b/browser/base/content/test/forms/browser_selectpopup_large.js
@@ -297,7 +297,7 @@ add_task(async function test_large_popup_in_small_window() {
     newWin,
     "resize",
     false,
-    e => {
+    () => {
       info(`Got resize event (innerHeight: ${newWin.innerHeight})`);
       return newWin.innerHeight <= 450;
     }
diff --git a/browser/base/content/test/forms/browser_selectpopup_minFontSize.js b/browser/base/content/test/forms/browser_selectpopup_minFontSize.js
index d240c2d2d0..522ed1ffcf 100644
--- a/browser/base/content/test/forms/browser_selectpopup_minFontSize.js
+++ b/browser/base/content/test/forms/browser_selectpopup_minFontSize.js
@@ -20,7 +20,7 @@ add_task(async function () {
       gBrowser,
       url,
     },
-    async function (browser) {
+    async function () {
       let popup = await openSelectPopup("click");
       let menuitems = popup.querySelectorAll("menuitem");
       is(
diff --git a/browser/base/content/test/forms/browser_selectpopup_text_transform.js b/browser/base/content/test/forms/browser_selectpopup_text_transform.js
index 671f39e2a6..04da532ddc 100644
--- a/browser/base/content/test/forms/browser_selectpopup_text_transform.js
+++ b/browser/base/content/test/forms/browser_selectpopup_text_transform.js
@@ -16,7 +16,7 @@ add_task(async function () {
       gBrowser,
       url,
     },
-    async function (browser) {
+    async function () {
       let popup = await openSelectPopup("click");
       let menuitems = popup.querySelectorAll("menuitem");
       is(menuitems[0].textContent, "abc", "Option text should be lowercase");
diff --git a/browser/base/content/test/forms/browser_selectpopup_user_input.js b/browser/base/content/test/forms/browser_selectpopup_user_input.js
index b3cdeaf7e6..028ceadf9a 100644
--- a/browser/base/content/test/forms/browser_selectpopup_user_input.js
+++ b/browser/base/content/test/forms/browser_selectpopup_user_input.js
@@ -71,7 +71,7 @@ async function testHandlingUserInputOnChange(aTriggerFn) {
 
 // This test checks if the change/click event is considered as user input event.
 add_task(async function test_handling_user_input_key() {
-  return testHandlingUserInputOnChange(async function (popup) {
+  return testHandlingUserInputOnChange(async function () {
     EventUtils.synthesizeKey("KEY_ArrowDown");
     await hideSelectPopup();
   });
diff --git a/browser/base/content/test/forms/browser_selectpopup_width.js b/browser/base/content/test/forms/browser_selectpopup_width.js
index d8f748fb18..0df0fd24ee 100644
--- a/browser/base/content/test/forms/browser_selectpopup_width.js
+++ b/browser/base/content/test/forms/browser_selectpopup_width.js
@@ -19,7 +19,7 @@ add_task(async function () {
       gBrowser,
       url,
     },
-    async function (browser) {
+    async function () {
       let popup = await openSelectPopup("click");
       let arrowSB = popup.shadowRoot.querySelector(".menupopup-arrowscrollbox");
       is(
diff --git a/browser/base/content/test/forms/browser_selectpopup_xhtml.js b/browser/base/content/test/forms/browser_selectpopup_xhtml.js
index 091649be89..27597eb5ac 100644
--- a/browser/base/content/test/forms/browser_selectpopup_xhtml.js
+++ b/browser/base/content/test/forms/browser_selectpopup_xhtml.js
@@ -21,7 +21,7 @@ add_task(async function () {
       gBrowser,
       url,
     },
-    async function (browser) {
+    async function () {
       let popup = await openSelectPopup("click");
       let menuitems = popup.querySelectorAll("menuitem");
       is(menuitems.length, 2, "Should've properly detected two menu items");
diff --git a/browser/base/content/test/fullscreen/browser_fullscreen_context_menu.js b/browser/base/content/test/fullscreen/browser_fullscreen_context_menu.js
index 9d9891acd2..3bca1a205d 100644
--- a/browser/base/content/test/fullscreen/browser_fullscreen_context_menu.js
+++ b/browser/base/content/test/fullscreen/browser_fullscreen_context_menu.js
@@ -51,7 +51,7 @@ async function testContextMenu() {
         window,
         "sizemodechange",
         false,
-        e => window.fullScreen
+        () => window.fullScreen
       ),
       BrowserTestUtils.waitForPopupEvent(contextMenu, "hidden"),
     ]);
@@ -96,7 +96,7 @@ async function testContextMenu() {
         window,
         "sizemodechange",
         false,
-        e => !window.fullScreen
+        () => !window.fullScreen
       ),
       BrowserTestUtils.waitForPopupEvent(contextMenu2, "hidden"),
     ]);
diff --git a/browser/base/content/test/fullscreen/browser_fullscreen_window_focus.js b/browser/base/content/test/fullscreen/browser_fullscreen_window_focus.js
index 5dd71e1a92..6e471e8124 100644
--- a/browser/base/content/test/fullscreen/browser_fullscreen_window_focus.js
+++ b/browser/base/content/test/fullscreen/browser_fullscreen_window_focus.js
@@ -74,7 +74,7 @@ async function testWindowElementFocus(isPopup) {
     false,
     async () => {
       info("Calling element.focus() on popup");
-      await ContentTask.spawn(tab.linkedBrowser, {}, async args => {
+      await ContentTask.spawn(tab.linkedBrowser, {}, async () => {
         await content.wrappedJSObject.sendMessage(
           content.wrappedJSObject.openedWindow,
           "elementfocus"
diff --git a/browser/base/content/test/fullscreen/head.js b/browser/base/content/test/fullscreen/head.js
index 4d5543461e..0d56c5a7c9 100644
--- a/browser/base/content/test/fullscreen/head.js
+++ b/browser/base/content/test/fullscreen/head.js
@@ -5,7 +5,7 @@ function waitForFullScreenState(browser, state, actionAfterFSEvent) {
   return new Promise(resolve => {
     let eventReceived = false;
 
-    let observe = (subject, topic, data) => {
+    let observe = () => {
       if (!eventReceived) {
         return;
       }
diff --git a/browser/base/content/test/general/browser.toml b/browser/base/content/test/general/browser.toml
index 6928ba2d4b..31d519d550 100644
--- a/browser/base/content/test/general/browser.toml
+++ b/browser/base/content/test/general/browser.toml
@@ -37,10 +37,6 @@ support-files = [
   "title_test.svg",
   "unknownContentType_file.pif",
   "unknownContentType_file.pif^headers^",
-  "video.ogg",
-  "web_video.html",
-  "web_video1.ogv",
-  "web_video1.ogv^headers^",
   "!/image/test/mochitest/blue.png",
   "!/toolkit/content/tests/browser/common/mockTransfer.js",
 ]
@@ -211,6 +207,7 @@ support-files = [
   "dummy.ics",
   "dummy.ics^headers^",
   "redirect_download.sjs",
+  "video.webm",
 ]
 # DO NOT ADD MORE TESTS HERE. USE A TOPICAL DIRECTORY INSTEAD.
 
@@ -435,9 +432,19 @@ skip-if = [
  "os == 'win' && debug",
  "os =='linux'", #Bug 1212419
 ]
+support-files = [
+  "web_video.html",
+  "web_video1.webm",
+  "web_video1.webm^headers^",
+]
 # DO NOT ADD MORE TESTS HERE. USE A TOPICAL DIRECTORY INSTEAD.
 
 ["browser_save_video_frame.js"]
+support-files = [
+  "web_video.html",
+  "web_video1.webm",
+  "web_video1.webm^headers^",
+]
 # DO NOT ADD MORE TESTS HERE. USE A TOPICAL DIRECTORY INSTEAD.
 
 ["browser_selectTabAtIndex.js"]
diff --git a/browser/base/content/test/general/browser_accesskeys.js b/browser/base/content/test/general/browser_accesskeys.js
index 0809553404..965da8d9df 100644
--- a/browser/base/content/test/general/browser_accesskeys.js
+++ b/browser/base/content/test/general/browser_accesskeys.js
@@ -122,7 +122,7 @@ add_task(async function () {
 function performAccessKey(browser, key) {
   return new Promise(resolve => {
     let removeFocus, removeKeyDown, removeKeyUp;
-    function callback(eventName, result) {
+    function callback() {
       removeFocus();
       removeKeyUp();
       removeKeyDown();
@@ -190,7 +190,7 @@ function performAccessKey(browser, key) {
 }
 
 // This version is used when a chrome element is expected to be found for an accesskey.
-async function performAccessKeyForChrome(key, inChild) {
+async function performAccessKeyForChrome(key) {
   let waitFocusChangePromise = BrowserTestUtils.waitForEvent(
     document,
     "focus",
diff --git a/browser/base/content/test/general/browser_alltabslistener.js b/browser/base/content/test/general/browser_alltabslistener.js
index c7829d16fe..fc950d6ce5 100644
--- a/browser/base/content/test/general/browser_alltabslistener.js
+++ b/browser/base/content/test/general/browser_alltabslistener.js
@@ -7,16 +7,9 @@ function getOriginalURL(request) {
 }
 
 var gFrontProgressListener = {
-  onProgressChange(
-    aWebProgress,
-    aRequest,
-    aCurSelfProgress,
-    aMaxSelfProgress,
-    aCurTotalProgress,
-    aMaxTotalProgress
-  ) {},
+  onProgressChange() {},
 
-  onStateChange(aWebProgress, aRequest, aStateFlags, aStatus) {
+  onStateChange(aWebProgress, aRequest, aStateFlags) {
     var url = getOriginalURL(aRequest);
     if (url == "about:blank") {
       return;
@@ -28,7 +21,7 @@ var gFrontProgressListener = {
     assertCorrectBrowserAndEventOrderForFront(state);
   },
 
-  onLocationChange(aWebProgress, aRequest, aLocationURI, aFlags) {
+  onLocationChange(aWebProgress, aRequest, aLocationURI) {
     var url = getOriginalURL(aRequest);
     if (url == "about:blank") {
       return;
@@ -64,7 +57,7 @@ function assertCorrectBrowserAndEventOrderForFront(aEventName) {
 }
 
 var gAllProgressListener = {
-  onStateChange(aBrowser, aWebProgress, aRequest, aStateFlags, aStatus) {
+  onStateChange(aBrowser, aWebProgress, aRequest, aStateFlags) {
     var url = getOriginalURL(aRequest);
     if (url == "about:blank") {
       // ignore initial about blank
diff --git a/browser/base/content/test/general/browser_beforeunload_duplicate_dialogs.js b/browser/base/content/test/general/browser_beforeunload_duplicate_dialogs.js
index 8eb07a863a..81ed5a1040 100644
--- a/browser/base/content/test/general/browser_beforeunload_duplicate_dialogs.js
+++ b/browser/base/content/test/general/browser_beforeunload_duplicate_dialogs.js
@@ -57,7 +57,7 @@ add_task(async function closeWindowWithMultipleTabsIncludingOneBeforeUnload() {
   );
   let windowClosedPromise = BrowserTestUtils.domWindowClosed(newWin);
   expectingDialog = true;
-  newWin.BrowserTryToCloseWindow();
+  newWin.BrowserCommands.tryToCloseWindow();
   await windowClosedPromise;
   ok(!expectingDialog, "There should have been a dialog.");
   ok(newWin.closed, "Window should be closed.");
@@ -71,7 +71,7 @@ add_task(async function closeWindoWithSingleTabTwice() {
   let windowClosedPromise = BrowserTestUtils.domWindowClosed(newWin);
   expectingDialog = true;
   wantToClose = false;
-  let firstDialogShownPromise = new Promise((resolve, reject) => {
+  let firstDialogShownPromise = new Promise(resolve => {
     resolveDialogPromise = resolve;
   });
   firstTab.closeButton.click();
diff --git a/browser/base/content/test/general/browser_bug356571.js b/browser/base/content/test/general/browser_bug356571.js
index aa3569c93d..69b45e040d 100644
--- a/browser/base/content/test/general/browser_bug356571.js
+++ b/browser/base/content/test/general/browser_bug356571.js
@@ -45,7 +45,7 @@ const kURIs = ["bad://www.mozilla.org/", kDummyPage, kDummyPage];
 
 var gProgressListener = {
   _runCount: 0,
-  onStateChange(aBrowser, aWebProgress, aRequest, aStateFlags, aStatus) {
+  onStateChange(aBrowser, aWebProgress, aRequest, aStateFlags) {
     if ((aStateFlags & kCompleteState) == kCompleteState) {
       if (++this._runCount != kURIs.length) {
         return;
diff --git a/browser/base/content/test/general/browser_bug417483.js b/browser/base/content/test/general/browser_bug417483.js
index 68e2e99511..28da91eea1 100644
--- a/browser/base/content/test/general/browser_bug417483.js
+++ b/browser/base/content/test/general/browser_bug417483.js
@@ -8,7 +8,7 @@ add_task(async function () {
   BrowserTestUtils.startLoadingURIString(gBrowser, htmlContent);
   await loadedPromise;
 
-  await SpecialPowers.spawn(gBrowser.selectedBrowser, [], async function (arg) {
+  await SpecialPowers.spawn(gBrowser.selectedBrowser, [], async function () {
     let frame = content.frames[0];
     let sel = frame.getSelection();
     let range = frame.document.createRange();
diff --git a/browser/base/content/test/general/browser_bug537013.js b/browser/base/content/test/general/browser_bug537013.js
index 5c871a759c..58bcec9754 100644
--- a/browser/base/content/test/general/browser_bug537013.js
+++ b/browser/base/content/test/general/browser_bug537013.js
@@ -15,7 +15,7 @@ var HasFindClipboard = Services.clipboard.isClipboardTypeSupported(
   Services.clipboard.kFindClipboard
 );
 
-function addTabWithText(aText, aCallback) {
+function addTabWithText(aText) {
   let newTab = BrowserTestUtils.addTab(
     gBrowser,
     "data:text/html;charset=utf-8,<h1 id='h1'>" + aText + "</h1>"
diff --git a/browser/base/content/test/general/browser_bug565575.js b/browser/base/content/test/general/browser_bug565575.js
index 6176c537e3..b974b17205 100644
--- a/browser/base/content/test/general/browser_bug565575.js
+++ b/browser/base/content/test/general/browser_bug565575.js
@@ -3,7 +3,7 @@ add_task(async function () {
 
   await BrowserTestUtils.openNewForegroundTab(
     gBrowser,
-    () => BrowserOpenTab(),
+    () => BrowserCommands.openTab(),
     false
   );
   ok(gURLBar.focused, "location bar is focused for a new tab");
diff --git a/browser/base/content/test/general/browser_bug567306.js b/browser/base/content/test/general/browser_bug567306.js
index 3d3e47e17d..24280371d8 100644
--- a/browser/base/content/test/general/browser_bug567306.js
+++ b/browser/base/content/test/general/browser_bug567306.js
@@ -10,7 +10,7 @@ add_task(async function () {
   let newwindow = await BrowserTestUtils.openNewBrowserWindow();
 
   let selectedBrowser = newwindow.gBrowser.selectedBrowser;
-  await new Promise((resolve, reject) => {
+  await new Promise(resolve => {
     BrowserTestUtils.waitForContentEvent(
       selectedBrowser,
       "pageshow",
diff --git a/browser/base/content/test/general/browser_bug609700.js b/browser/base/content/test/general/browser_bug609700.js
index 8195eba4ec..615b63c3d8 100644
--- a/browser/base/content/test/general/browser_bug609700.js
+++ b/browser/base/content/test/general/browser_bug609700.js
@@ -1,11 +1,7 @@
 function test() {
   waitForExplicitFinish();
 
-  Services.ww.registerNotification(function notification(
-    aSubject,
-    aTopic,
-    aData
-  ) {
+  Services.ww.registerNotification(function notification(aSubject, aTopic) {
     if (aTopic == "domwindowopened") {
       Services.ww.unregisterNotification(notification);
 
diff --git a/browser/base/content/test/general/browser_bug623893.js b/browser/base/content/test/general/browser_bug623893.js
index 79cd10c591..0f742a8e8e 100644
--- a/browser/base/content/test/general/browser_bug623893.js
+++ b/browser/base/content/test/general/browser_bug623893.js
@@ -38,7 +38,7 @@ async function promiseGetIndex(browser) {
   return shistory.index;
 }
 
-let duplicate = async function (delta, msg, cb) {
+let duplicate = async function (delta, msg) {
   var startIndex = await promiseGetIndex(gBrowser.selectedBrowser);
 
   duplicateTabIn(gBrowser.selectedTab, "tab", delta);
diff --git a/browser/base/content/test/general/browser_bug676619.js b/browser/base/content/test/general/browser_bug676619.js
index 80bbce8cb0..90dd8f4f7c 100644
--- a/browser/base/content/test/general/browser_bug676619.js
+++ b/browser/base/content/test/general/browser_bug676619.js
@@ -22,7 +22,7 @@ function waitForNewWindow() {
         var domwindow = aXULWindow.docShell.domWindow;
         domwindow.addEventListener("load", downloadOnLoad, true);
       },
-      onCloseWindow: aXULWindow => {},
+      onCloseWindow: () => {},
     };
 
     Services.wm.addListener(listener);
@@ -97,7 +97,7 @@ async function testLink(link, name) {
 }
 
 // Cross-origin URL does not trigger a download
-async function testLocation(link, url) {
+async function testLocation(link) {
   let tabPromise = BrowserTestUtils.waitForNewTab(gBrowser);
 
   SpecialPowers.spawn(gBrowser.selectedBrowser, [link], contentLink => {
@@ -116,8 +116,8 @@ async function runTest(url) {
   await BrowserTestUtils.browserLoaded(browser);
 
   await testLink("link1", "test.txt");
-  await testLink("link2", "video.ogg");
-  await testLink("link3", "just some video.ogg");
+  await testLink("link2", "video.webm");
+  await testLink("link3", "just some video.webm");
   await testLink("link4", "with-target.txt");
   await testLink("link5", "javascript.html");
   await testLink("link6", "test.blob");
@@ -132,8 +132,8 @@ async function runTest(url) {
   // Check that we enforce the correct extension if the website's
   // is bogus or missing. These extensions can differ slightly (ogx vs ogg,
   // htm vs html) on different OSes.
-  let oggExtension = getMIMEInfoForType("application/ogg").primaryExtension;
-  await testLink("link13", "no file extension." + oggExtension);
+  let webmExtension = getMIMEInfoForType("video/webm").primaryExtension;
+  await testLink("link13", "no file extension." + webmExtension);
 
   // See https://bugzilla.mozilla.org/show_bug.cgi?id=1690051#c8
   if (AppConstants.platform != "win") {
diff --git a/browser/base/content/test/general/browser_bug734076.js b/browser/base/content/test/general/browser_bug734076.js
index bd86f8e2b3..e9bec6834e 100644
--- a/browser/base/content/test/general/browser_bug734076.js
+++ b/browser/base/content/test/general/browser_bug734076.js
@@ -36,7 +36,7 @@ add_task(async function () {
         );
       },
       verify(browser) {
-        return SpecialPowers.spawn(browser, [], async function (arg) {
+        return SpecialPowers.spawn(browser, [], async function () {
           Assert.equal(
             content.document.body.textContent,
             "",
@@ -67,7 +67,7 @@ add_task(async function () {
         );
       },
       verify(browser) {
-        return SpecialPowers.spawn(browser, [], async function (arg) {
+        return SpecialPowers.spawn(browser, [], async function () {
           Assert.equal(
             content.document.body.textContent,
             "",
@@ -105,7 +105,7 @@ add_task(async function () {
         );
       },
       verify(browser) {
-        return SpecialPowers.spawn(browser, [], async function (arg) {
+        return SpecialPowers.spawn(browser, [], async function () {
           Assert.equal(
             content.document.body.textContent,
             "",
diff --git a/browser/base/content/test/general/browser_bug763468_perwindowpb.js b/browser/base/content/test/general/browser_bug763468_perwindowpb.js
index bed03561ca..05a7f90550 100644
--- a/browser/base/content/test/general/browser_bug763468_perwindowpb.js
+++ b/browser/base/content/test/general/browser_bug763468_perwindowpb.js
@@ -44,7 +44,7 @@ add_task(async function testPBNewTab() {
 
 async function openNewTab(aWindow, aExpectedURL) {
   // Open a new tab
-  aWindow.BrowserOpenTab();
+  aWindow.BrowserCommands.openTab();
   let browser = aWindow.gBrowser.selectedBrowser;
 
   // We're already loaded.
diff --git a/browser/base/content/test/general/browser_bug767836_perwindowpb.js b/browser/base/content/test/general/browser_bug767836_perwindowpb.js
index 7fcc6ad565..e237f1216d 100644
--- a/browser/base/content/test/general/browser_bug767836_perwindowpb.js
+++ b/browser/base/content/test/general/browser_bug767836_perwindowpb.js
@@ -59,7 +59,7 @@ add_task(async function test_newTabService() {
 
 async function openNewTab(aWindow, aExpectedURL) {
   // Open a new tab
-  aWindow.BrowserOpenTab();
+  aWindow.BrowserCommands.openTab();
   let browser = aWindow.gBrowser.selectedBrowser;
 
   // We're already loaded.
diff --git a/browser/base/content/test/general/browser_bug817947.js b/browser/base/content/test/general/browser_bug817947.js
index ea3c39222e..f83e07a9af 100644
--- a/browser/base/content/test/general/browser_bug817947.js
+++ b/browser/base/content/test/general/browser_bug817947.js
@@ -32,7 +32,7 @@ add_task(async () => {
   win.close();
 });
 
-async function preparePendingTab(aCallback) {
+async function preparePendingTab() {
   let tab = BrowserTestUtils.addTab(gBrowser, URL);
   await BrowserTestUtils.browserLoaded(tab.linkedBrowser);
 
diff --git a/browser/base/content/test/general/browser_clipboard.js b/browser/base/content/test/general/browser_clipboard.js
index a4c823969f..7820c4ec89 100644
--- a/browser/base/content/test/general/browser_clipboard.js
+++ b/browser/base/content/test/general/browser_clipboard.js
@@ -68,7 +68,7 @@ add_task(async function () {
       let selection = content.document.getSelection();
       selection.modify("move", "right", "line");
 
-      return new Promise((resolve, reject) => {
+      return new Promise(resolve => {
         content.addEventListener(
           "paste",
           event => {
@@ -130,7 +130,7 @@ add_task(async function () {
     selection.modify("extend", "left", "word");
     selection.modify("extend", "left", "character");
 
-    return new Promise((resolve, reject) => {
+    return new Promise(resolve => {
       content.addEventListener(
         "cut",
         event => {
@@ -157,7 +157,7 @@ add_task(async function () {
       let selection = content.document.getSelection();
       selection.modify("move", "left", "line");
 
-      return new Promise((resolve, reject) => {
+      return new Promise(resolve => {
         content.addEventListener(
           "paste",
           event => {
diff --git a/browser/base/content/test/general/browser_clipboard_pastefile.js b/browser/base/content/test/general/browser_clipboard_pastefile.js
index f034883ef2..6ef3edf30e 100644
--- a/browser/base/content/test/general/browser_clipboard_pastefile.js
+++ b/browser/base/content/test/general/browser_clipboard_pastefile.js
@@ -50,7 +50,7 @@ add_task(async function () {
   );
   let browser = tab.linkedBrowser;
 
-  let resultPromise = SpecialPowers.spawn(browser, [], function (arg) {
+  let resultPromise = SpecialPowers.spawn(browser, [], function () {
     return new Promise(resolve => {
       content.document.addEventListener("testresult", event => {
         resolve(event.detail.result);
@@ -73,7 +73,7 @@ add_task(async function () {
   document.documentElement.appendChild(input);
   input.focus();
 
-  await new Promise((resolve, reject) => {
+  await new Promise(resolve => {
     input.addEventListener(
       "paste",
       function (event) {
diff --git a/browser/base/content/test/general/browser_domFullscreen_fullscreenMode.js b/browser/base/content/test/general/browser_domFullscreen_fullscreenMode.js
index c96fa6cf7b..f44620c29e 100644
--- a/browser/base/content/test/general/browser_domFullscreen_fullscreenMode.js
+++ b/browser/base/content/test/general/browser_domFullscreen_fullscreenMode.js
@@ -149,7 +149,7 @@ add_task(async function () {
         gBrowser.selectedBrowser,
         FS_CHANGE_SIZE
       );
-      executeSoon(() => BrowserFullScreen());
+      executeSoon(() => BrowserCommands.fullScreen());
       await fullscreenPromise;
     }
   });
@@ -195,7 +195,7 @@ add_task(async function () {
     // dispatched synchronously, which would cause the event listener
     // miss that event and wait infinitely.
     fullscreenPromise = waitForFullscreenChanges(browser, FS_CHANGE_SIZE);
-    executeSoon(() => BrowserFullScreen());
+    executeSoon(() => BrowserCommands.fullScreen());
     contentStates = await fullscreenPromise;
     checkState({ inDOMFullscreen: false, inFullscreen: true }, contentStates);
 
@@ -228,7 +228,7 @@ add_task(async function () {
     if (window.fullScreen) {
       info("> Cleanup");
       fullscreenPromise = waitForFullscreenChanges(browser, FS_CHANGE_SIZE);
-      executeSoon(() => BrowserFullScreen());
+      executeSoon(() => BrowserCommands.fullScreen());
       await fullscreenPromise;
     }
   }
diff --git a/browser/base/content/test/general/browser_double_close_tab.js b/browser/base/content/test/general/browser_double_close_tab.js
index f5f2f1b6c7..6beea0f42b 100644
--- a/browser/base/content/test/general/browser_double_close_tab.js
+++ b/browser/base/content/test/general/browser_double_close_tab.js
@@ -18,7 +18,7 @@ function waitForDialog(callback) {
 
 function waitForDialogDestroyed(node, callback) {
   // Now listen for the dialog going away again...
-  let observer = new MutationObserver(function (muts) {
+  let observer = new MutationObserver(function () {
     if (!node.parentNode) {
       ok(true, "Dialog is gone");
       done();
diff --git a/browser/base/content/test/general/browser_focusonkeydown.js b/browser/base/content/test/general/browser_focusonkeydown.js
index 9cf1f113f5..53919bc1b3 100644
--- a/browser/base/content/test/general/browser_focusonkeydown.js
+++ b/browser/base/content/test/general/browser_focusonkeydown.js
@@ -20,7 +20,7 @@ add_task(async function () {
 
   gURLBar.addEventListener(
     "keydown",
-    function (event) {
+    function () {
       gBrowser.selectedBrowser.focus();
     },
     { capture: true, once: true }
diff --git a/browser/base/content/test/general/browser_fullscreen-window-open.js b/browser/base/content/test/general/browser_fullscreen-window-open.js
index 2b21e34e92..be1d2ca3a3 100644
--- a/browser/base/content/test/general/browser_fullscreen-window-open.js
+++ b/browser/base/content/test/general/browser_fullscreen-window-open.js
@@ -26,14 +26,14 @@ async function test() {
   await promiseTabLoadEvent(newBrowser.selectedTab, gHttpTestRoot + TEST_FILE);
 
   // Enter browser fullscreen mode.
-  newWin.BrowserFullScreen();
+  newWin.BrowserCommands.fullScreen();
 
   runNextTest();
 }
 
 registerCleanupFunction(async function () {
   // Exit browser fullscreen mode.
-  newWin.BrowserFullScreen();
+  newWin.BrowserCommands.fullScreen();
 
   await BrowserTestUtils.closeWindow(newWin);
 
@@ -336,7 +336,7 @@ WindowListener.prototype = {
     Services.wm.removeListener(this);
 
     let domwindow = aXULWindow.docShell.domWindow;
-    let onLoad = aEvent => {
+    let onLoad = () => {
       is(
         domwindow.document.location.href,
         this.test_url,
@@ -361,6 +361,6 @@ WindowListener.prototype = {
     };
     domwindow.addEventListener("load", onLoad, true);
   },
-  onCloseWindow(aXULWindow) {},
+  onCloseWindow() {},
   QueryInterface: ChromeUtils.generateQI(["nsIWindowMediatorListener"]),
 };
diff --git a/browser/base/content/test/general/browser_invalid_uri_back_forward_manipulation.js b/browser/base/content/test/general/browser_invalid_uri_back_forward_manipulation.js
index 1624a1514d..fae1130685 100644
--- a/browser/base/content/test/general/browser_invalid_uri_back_forward_manipulation.js
+++ b/browser/base/content/test/general/browser_invalid_uri_back_forward_manipulation.js
@@ -33,7 +33,7 @@ add_task(async function checkBackFromInvalidURI() {
       false,
       // Be paranoid we *are* actually seeing this other page load, not some kind of race
       // for if/when we do start firing pageshow for the error page...
-      function (e) {
+      function () {
         return gBrowser.currentURI.spec == "about:robots";
       }
     );
diff --git a/browser/base/content/test/general/browser_newWindowDrop.js b/browser/base/content/test/general/browser_newWindowDrop.js
index 3e41b0d6ac..445999befd 100644
--- a/browser/base/content/test/general/browser_newWindowDrop.js
+++ b/browser/base/content/test/general/browser_newWindowDrop.js
@@ -184,7 +184,7 @@ function dropText(text, expectedURLs, ignoreFirstWindow = false) {
   );
 }
 
-async function drop(dragData, expectedURLs, ignoreFirstWindow = false) {
+async function drop(dragData, expectedURLs) {
   let dragDataString = JSON.stringify(dragData);
   info(
     `Starting test for dragData:${dragDataString}; expectedURLs.length:${expectedURLs.length}`
diff --git a/browser/base/content/test/general/browser_plainTextLinks.js b/browser/base/content/test/general/browser_plainTextLinks.js
index 706f21387c..44c9b8422b 100644
--- a/browser/base/content/test/general/browser_plainTextLinks.js
+++ b/browser/base/content/test/general/browser_plainTextLinks.js
@@ -19,7 +19,7 @@ add_task(async function () {
   await SimpleTest.promiseFocus(gBrowser.selectedBrowser);
 
   // Initial setup of the content area.
-  await SpecialPowers.spawn(gBrowser.selectedBrowser, [], async function (arg) {
+  await SpecialPowers.spawn(gBrowser.selectedBrowser, [], async function () {
     let doc = content.document;
     let range = doc.createRange();
     let selection = content.getSelection();
diff --git a/browser/base/content/test/general/browser_private_no_prompt.js b/browser/base/content/test/general/browser_private_no_prompt.js
index d8c9f8e7b5..80ba0ca746 100644
--- a/browser/base/content/test/general/browser_private_no_prompt.js
+++ b/browser/base/content/test/general/browser_private_no_prompt.js
@@ -3,8 +3,8 @@ function test() {
   var privateWin = OpenBrowserWindow({ private: true });
 
   whenDelayedStartupFinished(privateWin, function () {
-    privateWin.BrowserOpenTab();
-    privateWin.BrowserTryToCloseWindow();
+    privateWin.BrowserCommands.openTab();
+    privateWin.BrowserCommands.tryToCloseWindow();
     ok(true, "didn't prompt");
 
     executeSoon(finish);
diff --git a/browser/base/content/test/general/browser_remoteTroubleshoot.js b/browser/base/content/test/general/browser_remoteTroubleshoot.js
index 84722b2603..55627f0b28 100644
--- a/browser/base/content/test/general/browser_remoteTroubleshoot.js
+++ b/browser/base/content/test/general/browser_remoteTroubleshoot.js
@@ -19,9 +19,9 @@ const TEST_URI_GOOD_OBJECT = Services.io.newURI(
 
 // Creates a one-shot web-channel for the test data to be sent back from the test page.
 function promiseChannelResponse(channelID, originOrPermission) {
-  return new Promise((resolve, reject) => {
+  return new Promise(resolve => {
     let channel = new WebChannel(channelID, originOrPermission);
-    channel.listen((id, data, target) => {
+    channel.listen((id, data) => {
       channel.stopListening();
       resolve(data);
     });
diff --git a/browser/base/content/test/general/browser_save_link-perwindowpb.js b/browser/base/content/test/general/browser_save_link-perwindowpb.js
index d5a0eef86c..29fd54d2d9 100644
--- a/browser/base/content/test/general/browser_save_link-perwindowpb.js
+++ b/browser/base/content/test/general/browser_save_link-perwindowpb.js
@@ -68,7 +68,7 @@ function triggerSave(aWindow, aCallback) {
     info("popup hidden");
   }
 
-  function onTransferComplete(aWindow2, downloadSuccess, destDir) {
+  function onTransferComplete(aWindow2, downloadSuccess) {
     ok(downloadSuccess, "Link should have been downloaded successfully");
     aWindow2.close();
 
@@ -118,7 +118,7 @@ function test() {
     info("Finished running the cleanup code");
   });
 
-  function observer(subject, topic, state) {
+  function observer(subject, topic) {
     info("observer called with " + topic);
     if (topic == "http-on-modify-request") {
       onModifyRequest(subject);
diff --git a/browser/base/content/test/general/browser_save_link_when_window_navigates.js b/browser/base/content/test/general/browser_save_link_when_window_navigates.js
index 1c68b91ddf..e5c7fa76b2 100644
--- a/browser/base/content/test/general/browser_save_link_when_window_navigates.js
+++ b/browser/base/content/test/general/browser_save_link_when_window_navigates.js
@@ -70,7 +70,7 @@ function triggerSave(aWindow, aCallback) {
     info("done mockTransferCallback");
   };
 
-  function onUCTDialog(dialog) {
+  function onUCTDialog() {
     SpecialPowers.spawn(testBrowser, [], async () => {
       content.document.querySelector("iframe").remove();
     }).then(() => executeSoon(continueDownloading));
@@ -104,7 +104,7 @@ var windowObserver = {
     }
     this._callback = aCallback;
   },
-  observe(aSubject, aTopic, aData) {
+  observe(aSubject, aTopic) {
     if (aTopic != "domwindowopened") {
       return;
     }
@@ -113,7 +113,7 @@ var windowObserver = {
 
     win.addEventListener(
       "load",
-      function (event) {
+      function () {
         if (win.location == UCT_URI) {
           SimpleTest.executeSoon(function () {
             if (windowObserver._callback) {
diff --git a/browser/base/content/test/general/browser_save_private_link_perwindowpb.js b/browser/base/content/test/general/browser_save_private_link_perwindowpb.js
index 42632bdc5a..1312c7b954 100644
--- a/browser/base/content/test/general/browser_save_private_link_perwindowpb.js
+++ b/browser/base/content/test/general/browser_save_private_link_perwindowpb.js
@@ -12,9 +12,9 @@ function createTemporarySaveDirectory() {
 }
 
 function promiseNoCacheEntry(filename) {
-  return new Promise((resolve, reject) => {
+  return new Promise(resolve => {
     Visitor.prototype = {
-      onCacheStorageInfo(num, consumption) {
+      onCacheStorageInfo(num) {
         info("disk storage contains " + num + " entries");
       },
       onCacheEntryInfo(uri) {
@@ -40,7 +40,7 @@ function promiseNoCacheEntry(filename) {
 }
 
 function promiseImageDownloaded() {
-  return new Promise((resolve, reject) => {
+  return new Promise(resolve => {
     let fileName;
     let MockFilePicker = SpecialPowers.MockFilePicker;
     MockFilePicker.init(window.browsingContext);
diff --git a/browser/base/content/test/general/browser_save_video.js b/browser/base/content/test/general/browser_save_video.js
index d8dc5c6e2e..f0450ac1fa 100644
--- a/browser/base/content/test/general/browser_save_video.js
+++ b/browser/base/content/test/general/browser_save_video.js
@@ -52,7 +52,7 @@ add_task(async function () {
 
       is(
         fileName,
-        "web-video1-expectedName.ogv",
+        "web-video1-expectedName.webm",
         "Video file name is correctly retrieved from Content-Disposition http header"
       );
       resolve();
diff --git a/browser/base/content/test/general/browser_tabfocus.js b/browser/base/content/test/general/browser_tabfocus.js
index b057a504e5..7cc9158084 100644
--- a/browser/base/content/test/general/browser_tabfocus.js
+++ b/browser/base/content/test/general/browser_tabfocus.js
@@ -322,7 +322,7 @@ add_task(async function () {
     "tab change when selected tab element was focused"
   );
 
-  let switchWaiter = new Promise((resolve, reject) => {
+  let switchWaiter = new Promise(resolve => {
     gBrowser.addEventListener(
       "TabSwitchDone",
       function () {
@@ -516,7 +516,7 @@ add_task(async function () {
   // now go back again
   gURLBar.focus();
 
-  await new Promise((resolve, reject) => {
+  await new Promise(resolve => {
     BrowserTestUtils.waitForContentEvent(
       window.gBrowser.selectedBrowser,
       "pageshow",
diff --git a/browser/base/content/test/general/browser_tabs_owner.js b/browser/base/content/test/general/browser_tabs_owner.js
index 4a32da12f1..e214b861e8 100644
--- a/browser/base/content/test/general/browser_tabs_owner.js
+++ b/browser/base/content/test/general/browser_tabs_owner.js
@@ -8,13 +8,13 @@ function test() {
   is(gBrowser.tabs.length, 4, "4 tabs are open");
 
   owner = gBrowser.selectedTab = gBrowser.tabs[2];
-  BrowserOpenTab();
+  BrowserCommands.openTab();
   is(gBrowser.selectedTab, gBrowser.tabs[4], "newly opened tab is selected");
   gBrowser.removeCurrentTab();
   is(gBrowser.selectedTab, owner, "owner is selected");
 
   owner = gBrowser.selectedTab;
-  BrowserOpenTab();
+  BrowserCommands.openTab();
   gBrowser.selectedTab = gBrowser.tabs[1];
   gBrowser.selectedTab = gBrowser.tabs[4];
   gBrowser.removeCurrentTab();
@@ -25,7 +25,7 @@ function test() {
   );
 
   owner = gBrowser.selectedTab;
-  BrowserOpenTab();
+  BrowserCommands.openTab();
   gBrowser.moveTabTo(gBrowser.selectedTab, 0);
   gBrowser.removeCurrentTab();
   is(
diff --git a/browser/base/content/test/general/browser_viewSourceInTabOnViewSource.js b/browser/base/content/test/general/browser_viewSourceInTabOnViewSource.js
index 6c62670e6f..26c040324a 100644
--- a/browser/base/content/test/general/browser_viewSourceInTabOnViewSource.js
+++ b/browser/base/content/test/general/browser_viewSourceInTabOnViewSource.js
@@ -1,7 +1,7 @@
 function wait_while_tab_is_busy() {
   return new Promise(resolve => {
     let progressListener = {
-      onStateChange(aWebProgress, aRequest, aStateFlags, aStatus) {
+      onStateChange(aWebProgress, aRequest, aStateFlags) {
         if (aStateFlags & Ci.nsIWebProgressListener.STATE_STOP) {
           gBrowser.removeProgressListener(this);
           setTimeout(resolve, 0);
@@ -27,7 +27,7 @@ var with_new_tab_opened = async function (options, taskFn) {
 };
 
 add_task(async function test_regular_page() {
-  function test_expect_view_source_enabled(browser) {
+  function test_expect_view_source_enabled() {
     for (let element of [...XULBrowserWindow._elementsForViewSource]) {
       ok(!element.hasAttribute("disabled"), "View Source should be enabled");
     }
@@ -44,7 +44,7 @@ add_task(async function test_regular_page() {
 });
 
 add_task(async function test_view_source_page() {
-  function test_expect_view_source_disabled(browser) {
+  function test_expect_view_source_disabled() {
     for (let element of [...XULBrowserWindow._elementsForViewSource]) {
       ok(element.hasAttribute("disabled"), "View Source should be disabled");
     }
diff --git a/browser/base/content/test/general/browser_zbug569342.js b/browser/base/content/test/general/browser_zbug569342.js
index 4aa6bfbb9c..0c30ff3d1d 100644
--- a/browser/base/content/test/general/browser_zbug569342.js
+++ b/browser/base/content/test/general/browser_zbug569342.js
@@ -57,7 +57,7 @@ function testFindDisabled(url) {
 }
 
 async function testFindEnabled(url) {
-  return BrowserTestUtils.withNewTab(url, async function (browser) {
+  return BrowserTestUtils.withNewTab(url, async function () {
     ok(
       !document.getElementById("cmd_find").getAttribute("disabled"),
       "Find command should not be disabled"
diff --git a/browser/base/content/test/general/download_page.html b/browser/base/content/test/general/download_page.html
index 300bacdb72..625ff46aab 100644
--- a/browser/base/content/test/general/download_page.html
+++ b/browser/base/content/test/general/download_page.html
@@ -13,10 +13,10 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=676619
     <ul>
         <li><a href="download_page_1.txt"
                 download="test.txt" id="link1">Download "test.txt"</a></li>
-        <li><a href="video.ogg"
-                download id="link2">Download "video.ogg"</a></li>
-        <li><a href="video.ogg"
-                download="just some video.ogg" id="link3">Download "just some video.ogg"</a></li>
+        <li><a href="video.webm"
+                download id="link2">Download "video.webm"</a></li>
+        <li><a href="video.webm"
+                download="just some video.webm" id="link3">Download "just some video.webm"</a></li>
         <li><a href="download_page_2.txt"
                 download="with-target.txt" id="link4">Download "with-target.txt"</a></li>
         <li><a href="javascript:(1+2)+''"
@@ -33,7 +33,7 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=676619
             download="download_page_4.txt" id="link11">Download "download_page_4.txt"</a></li>
         <li><a href="http://example.com/"
             download="example.com" id="link12" target="_blank">Download "example.com"</a></li>
-        <li><a href="video.ogg"
+        <li><a href="video.webm"
             download="no file extension" id="link13">Download "force extension"</a></li>
         <li><a href="dummy.ics"
             download="dummy.not-ics" id="link14">Download "dummy.not-ics"</a></li>
@@ -64,7 +64,7 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=676619
               "wrong-file-name", {type: "application/x-some-file"}));
         document.getElementById("link7").href = fileURL;
 
-        window.addEventListener("beforeunload", function(evt) {
+        window.addEventListener("beforeunload", function() {
             document.getElementById("unload-flag").textContent = "Fail";
         });
     </script>
diff --git a/browser/base/content/test/general/head.js b/browser/base/content/test/general/head.js
index f7b4a0d93b..94f60f2936 100644
--- a/browser/base/content/test/general/head.js
+++ b/browser/base/content/test/general/head.js
@@ -165,7 +165,7 @@ function promiseOpenAndLoadWindow(aOptions, aWaitForDelayedStartup = false) {
   return new Promise(resolve => {
     let win = OpenBrowserWindow(aOptions);
     if (aWaitForDelayedStartup) {
-      Services.obs.addObserver(function onDS(aSubject, aTopic, aData) {
+      Services.obs.addObserver(function onDS(aSubject) {
         if (aSubject != win) {
           return;
         }
@@ -185,7 +185,7 @@ function promiseOpenAndLoadWindow(aOptions, aWaitForDelayedStartup = false) {
 }
 
 async function whenNewTabLoaded(aWindow, aCallback) {
-  aWindow.BrowserOpenTab();
+  aWindow.BrowserCommands.openTab();
 
   let expectedURL = AboutNewTab.newTabURL;
   let browser = aWindow.gBrowser.selectedBrowser;
diff --git a/browser/base/content/test/general/video.ogg b/browser/base/content/test/general/video.ogg
deleted file mode 100644
index ac7ece3519..0000000000
Binary files a/browser/base/content/test/general/video.ogg and /dev/null differ
diff --git a/browser/base/content/test/general/video.webm b/browser/base/content/test/general/video.webm
new file mode 100644
index 0000000000..0ca38d3cf0
Binary files /dev/null and b/browser/base/content/test/general/video.webm differ
diff --git a/browser/base/content/test/general/web_video.html b/browser/base/content/test/general/web_video.html
index 467fb0ce1c..e03c85d1dc 100644
--- a/browser/base/content/test/general/web_video.html
+++ b/browser/base/content/test/general/web_video.html
@@ -5,6 +5,6 @@
   <body>
     This document has some web video in it.
     <br>
-    <video src="web_video1.ogv" id="video1"> </video>
+    <video src="web_video1.webm" id="video1"> </video>
   </body>
 </html>
diff --git a/browser/base/content/test/general/web_video1.ogv b/browser/base/content/test/general/web_video1.ogv
deleted file mode 100644
index 093158432a..0000000000
Binary files a/browser/base/content/test/general/web_video1.ogv and /dev/null differ
diff --git a/browser/base/content/test/general/web_video1.ogv^headers^ b/browser/base/content/test/general/web_video1.ogv^headers^
deleted file mode 100644
index 4511e92552..0000000000
--- a/browser/base/content/test/general/web_video1.ogv^headers^
+++ /dev/null
@@ -1,3 +0,0 @@
-Content-Disposition: filename="web-video1-expectedName.ogv" 
-Content-Type: video/ogg
-
diff --git a/browser/base/content/test/general/web_video1.webm b/browser/base/content/test/general/web_video1.webm
new file mode 100644
index 0000000000..2c9d7dad8d
Binary files /dev/null and b/browser/base/content/test/general/web_video1.webm differ
diff --git a/browser/base/content/test/general/web_video1.webm^headers^ b/browser/base/content/test/general/web_video1.webm^headers^
new file mode 100644
index 0000000000..d027132ea2
--- /dev/null
+++ b/browser/base/content/test/general/web_video1.webm^headers^
@@ -0,0 +1,3 @@
+Content-Disposition: filename="web-video1-expectedName.webm" 
+Content-Type: video/webm
+
diff --git a/browser/base/content/test/historySwipeAnimation/browser_historySwipeAnimation.js b/browser/base/content/test/historySwipeAnimation/browser_historySwipeAnimation.js
index a5910964e7..eeeb7e8b9e 100644
--- a/browser/base/content/test/historySwipeAnimation/browser_historySwipeAnimation.js
+++ b/browser/base/content/test/historySwipeAnimation/browser_historySwipeAnimation.js
@@ -5,7 +5,7 @@
 function test() {
   waitForExplicitFinish();
 
-  BrowserOpenTab();
+  BrowserCommands.openTab();
   let tab = gBrowser.selectedTab;
   registerCleanupFunction(function () {
     gBrowser.removeTab(tab);
diff --git a/browser/base/content/test/keyboard/browser_toolbarButtonKeyPress.js b/browser/base/content/test/keyboard/browser_toolbarButtonKeyPress.js
index 8640716bab..ef92d4c528 100644
--- a/browser/base/content/test/keyboard/browser_toolbarButtonKeyPress.js
+++ b/browser/base/content/test/keyboard/browser_toolbarButtonKeyPress.js
@@ -12,7 +12,7 @@ const kDevPanelID = "PanelUI-developer-tools";
 function waitForLocationChange() {
   let promise = new Promise(resolve => {
     let wpl = {
-      onLocationChange(aWebProgress, aRequest, aLocation) {
+      onLocationChange() {
         gBrowser.removeProgressListener(wpl);
         resolve();
       },
@@ -213,31 +213,23 @@ add_task(async function testSidebarsButtonPress() {
 // This is an image with a click handler on its parent and no command handler,
 // but the toolbar keyboard navigation code should handle keyboard activation.
 add_task(async function testBookmarkButtonPress() {
-  await BrowserTestUtils.withNewTab(
-    "https://example.com",
-    async function (aBrowser) {
-      let button = document.getElementById("star-button-box");
-      StarUI._createPanelIfNeeded();
-      let panel = document.getElementById("editBookmarkPanel");
-      let focused = BrowserTestUtils.waitForEvent(panel, "focus", true);
-      // The button ignores activation while the bookmarked status is being
-      // updated. So, wait for it to finish updating.
-      await TestUtils.waitForCondition(
-        () => BookmarkingUI.status != BookmarkingUI.STATUS_UPDATING
-      );
-      await focusAndActivateElement(button, () =>
-        EventUtils.synthesizeKey(" ")
-      );
-      await focused;
-      ok(
-        true,
-        "Focus inside edit bookmark panel after Bookmark button pressed"
-      );
-      let hidden = BrowserTestUtils.waitForEvent(panel, "popuphidden");
-      EventUtils.synthesizeKey("KEY_Escape");
-      await hidden;
-    }
-  );
+  await BrowserTestUtils.withNewTab("https://example.com", async function () {
+    let button = document.getElementById("star-button-box");
+    StarUI._createPanelIfNeeded();
+    let panel = document.getElementById("editBookmarkPanel");
+    let focused = BrowserTestUtils.waitForEvent(panel, "focus", true);
+    // The button ignores activation while the bookmarked status is being
+    // updated. So, wait for it to finish updating.
+    await TestUtils.waitForCondition(
+      () => BookmarkingUI.status != BookmarkingUI.STATUS_UPDATING
+    );
+    await focusAndActivateElement(button, () => EventUtils.synthesizeKey(" "));
+    await focused;
+    ok(true, "Focus inside edit bookmark panel after Bookmark button pressed");
+    let hidden = BrowserTestUtils.waitForEvent(panel, "popuphidden");
+    EventUtils.synthesizeKey("KEY_Escape");
+    await hidden;
+  });
 });
 
 // Test activation of the Bookmarks Menu button from the keyboard.
@@ -302,33 +294,24 @@ add_task(async function testDownloadsButtonPress() {
 // with a browser element to embed the pocket UI into it.
 // The Pocket panel should appear and focus should move inside it.
 add_task(async function testPocketButtonPress() {
-  await BrowserTestUtils.withNewTab(
-    "https://example.com",
-    async function (aBrowser) {
-      let button = document.getElementById("save-to-pocket-button");
-      // The panel is created on the fly, so we can't simply wait for focus
-      // inside it.
-      let showing = BrowserTestUtils.waitForEvent(
-        document,
-        "popupshowing",
-        true
-      );
-      await focusAndActivateElement(button, () =>
-        EventUtils.synthesizeKey(" ")
-      );
-      let event = await showing;
-      let panel = event.target;
-      is(panel.id, "customizationui-widget-panel");
-      let focused = BrowserTestUtils.waitForEvent(panel, "focus", true);
-      await focused;
-      is(
-        document.activeElement.tagName,
-        "browser",
-        "Focus inside Pocket panel after Bookmark button pressed"
-      );
-      let hidden = BrowserTestUtils.waitForEvent(panel, "popuphidden");
-      EventUtils.synthesizeKey("KEY_Escape");
-      await hidden;
-    }
-  );
+  await BrowserTestUtils.withNewTab("https://example.com", async function () {
+    let button = document.getElementById("save-to-pocket-button");
+    // The panel is created on the fly, so we can't simply wait for focus
+    // inside it.
+    let showing = BrowserTestUtils.waitForEvent(document, "popupshowing", true);
+    await focusAndActivateElement(button, () => EventUtils.synthesizeKey(" "));
+    let event = await showing;
+    let panel = event.target;
+    is(panel.id, "customizationui-widget-panel");
+    let focused = BrowserTestUtils.waitForEvent(panel, "focus", true);
+    await focused;
+    is(
+      document.activeElement.tagName,
+      "browser",
+      "Focus inside Pocket panel after Bookmark button pressed"
+    );
+    let hidden = BrowserTestUtils.waitForEvent(panel, "popuphidden");
+    EventUtils.synthesizeKey("KEY_Escape");
+    await hidden;
+  });
 });
diff --git a/browser/base/content/test/metaTags/browser_bad_meta_tags.js b/browser/base/content/test/metaTags/browser_bad_meta_tags.js
index 00cc128ec0..aa025725dc 100644
--- a/browser/base/content/test/metaTags/browser_bad_meta_tags.js
+++ b/browser/base/content/test/metaTags/browser_bad_meta_tags.js
@@ -9,11 +9,12 @@ const TEST_PATH =
   ) + "bad_meta_tags.html";
 
 /**
- * This tests that with the page bad_meta_tags.html, ContentMetaHandler.jsm parses
- * out the meta tags available and does not store content provided by a malformed
- * meta tag. In this case the best defined meta tags are malformed, so here we
- * test that we store the next best ones - "description" and "twitter:image". The
- * list of meta tags and order of preference is found in ContentMetaHandler.jsm.
+ * This tests that with the page bad_meta_tags.html, ContentMetaHandler.sys.mjs
+ * parses out the meta tags available and does not store content provided by a
+ * malformed meta tag. In this case the best defined meta tags are malformed, so
+ * here we test that we store the next best ones - "description" and "twitter:image".
+ * The list of meta tags and order of preference is found in
+ * ContentMetaHandler.sys.mjs.
  */
 add_task(async function test_bad_meta_tags() {
   const tab = await BrowserTestUtils.openNewForegroundTab(gBrowser, TEST_PATH);
diff --git a/browser/base/content/test/metaTags/browser_meta_tags.js b/browser/base/content/test/metaTags/browser_meta_tags.js
index 380a71214c..870860dc18 100644
--- a/browser/base/content/test/metaTags/browser_meta_tags.js
+++ b/browser/base/content/test/metaTags/browser_meta_tags.js
@@ -8,11 +8,11 @@ const TEST_PATH =
     "https://example.com"
   ) + "meta_tags.html";
 /**
- * This tests that with the page meta_tags.html, ContentMetaHandler.jsm parses
- * out the meta tags avilable and only stores the best one for description and
- * one for preview image url. In the case of this test, the best defined meta
+ * This tests that with the page meta_tags.html, ContentMetaHandler.sys.mjs
+ * parses out the meta tags avilable and only stores the best one for description
+ * and one for preview image url. In the case of this test, the best defined meta
  * tags are "og:description" and "og:image:secure_url". The list of meta tags
- * and order of preference is found in ContentMetaHandler.jsm.
+ * and order of preference is found in ContentMetaHandler.sys.mjs.
  */
 add_task(async function test_metadata() {
   const tab = await BrowserTestUtils.openNewForegroundTab(gBrowser, TEST_PATH);
diff --git a/browser/base/content/test/outOfProcess/browser_basic_outofprocess.js b/browser/base/content/test/outOfProcess/browser_basic_outofprocess.js
index 50914a286c..41fc96986e 100644
--- a/browser/base/content/test/outOfProcess/browser_basic_outofprocess.js
+++ b/browser/base/content/test/outOfProcess/browser_basic_outofprocess.js
@@ -121,12 +121,9 @@ add_task(async function test_subframes_function() {
   let browser = tab.linkedBrowser;
 
   let counter = 0;
-  let browsingContexts = await initChildFrames(
-    browser,
-    function (browsingContext) {
-      return "<p>Text " + ++counter + "</p>";
-    }
-  );
+  let browsingContexts = await initChildFrames(browser, function () {
+    return "<p>Text " + ++counter + "</p>";
+  });
 
   is(
     counter,
diff --git a/browser/base/content/test/pageActions/head.js b/browser/base/content/test/pageActions/head.js
index cd269bf9b5..370f01734c 100644
--- a/browser/base/content/test/pageActions/head.js
+++ b/browser/base/content/test/pageActions/head.js
@@ -124,7 +124,7 @@ async function promiseAnimationFrame(win = window) {
 
 async function promisePopupNotShown(id, win = window) {
   let deferred = Promise.withResolvers();
-  function listener(e) {
+  function listener() {
     deferred.reject("Unexpected popupshown");
   }
   let panel = win.document.getElementById(id);
diff --git a/browser/base/content/test/pageinfo/browser.toml b/browser/base/content/test/pageinfo/browser.toml
index ae70eb68ff..9e14392450 100644
--- a/browser/base/content/test/pageinfo/browser.toml
+++ b/browser/base/content/test/pageinfo/browser.toml
@@ -5,7 +5,7 @@ support-files = [
   "image.html",
   "../general/audio.ogg",
   "../general/moz.png",
-  "../general/video.ogg",
+  "../general/video.webm",
 ]
 
 ["browser_pageinfo_iframe_media.js"]
diff --git a/browser/base/content/test/pageinfo/browser_pageinfo_firstPartyIsolation.js b/browser/base/content/test/pageinfo/browser_pageinfo_firstPartyIsolation.js
index 354e85a241..dbd5d8fe25 100644
--- a/browser/base/content/test/pageinfo/browser_pageinfo_firstPartyIsolation.js
+++ b/browser/base/content/test/pageinfo/browser_pageinfo_firstPartyIsolation.js
@@ -76,7 +76,7 @@ async function test() {
   // Pass a dummy imageElement, if there isn't an imageElement, pageInfo.js
   // will do a preview, however this sometimes will cause intermittent failures,
   // see bug 1403365.
-  let pageInfo = BrowserPageInfo(url, "mediaTab", {});
+  let pageInfo = BrowserCommands.pageInfo(url, "mediaTab", {});
   info("waitForEvent pageInfo");
   await BrowserTestUtils.waitForEvent(pageInfo, "load");
 
diff --git a/browser/base/content/test/pageinfo/browser_pageinfo_iframe_media.js b/browser/base/content/test/pageinfo/browser_pageinfo_iframe_media.js
index 7550379ad1..3dad0a50f3 100644
--- a/browser/base/content/test/pageinfo/browser_pageinfo_iframe_media.js
+++ b/browser/base/content/test/pageinfo/browser_pageinfo_iframe_media.js
@@ -10,7 +10,7 @@ add_task(async function test_all_images_mentioned() {
   await BrowserTestUtils.withNewTab(
     TEST_PATH + "iframes.html",
     async function () {
-      let pageInfo = BrowserPageInfo(
+      let pageInfo = BrowserCommands.pageInfo(
         gBrowser.selectedBrowser.currentURI.spec,
         "mediaTab"
       );
diff --git a/browser/base/content/test/pageinfo/browser_pageinfo_image_info.js b/browser/base/content/test/pageinfo/browser_pageinfo_image_info.js
index 374cd5f032..0fea68d640 100644
--- a/browser/base/content/test/pageinfo/browser_pageinfo_image_info.js
+++ b/browser/base/content/test/pageinfo/browser_pageinfo_image_info.js
@@ -28,7 +28,7 @@ add_task(async function () {
     };
   });
 
-  let pageInfo = BrowserPageInfo(
+  let pageInfo = BrowserCommands.pageInfo(
     browser.currentURI.spec,
     "mediaTab",
     imageInfo
diff --git a/browser/base/content/test/pageinfo/browser_pageinfo_images.js b/browser/base/content/test/pageinfo/browser_pageinfo_images.js
index e1f71204d0..c356c1c690 100644
--- a/browser/base/content/test/pageinfo/browser_pageinfo_images.js
+++ b/browser/base/content/test/pageinfo/browser_pageinfo_images.js
@@ -10,7 +10,7 @@ add_task(async function test_all_images_mentioned() {
   await BrowserTestUtils.withNewTab(
     TEST_PATH + "all_images.html",
     async function () {
-      let pageInfo = BrowserPageInfo(
+      let pageInfo = BrowserCommands.pageInfo(
         gBrowser.selectedBrowser.currentURI.spec,
         "mediaTab"
       );
@@ -97,7 +97,7 @@ add_task(async function test_image_size() {
   await BrowserTestUtils.withNewTab(
     TEST_PATH + "all_images.html",
     async function () {
-      let pageInfo = BrowserPageInfo(
+      let pageInfo = BrowserCommands.pageInfo(
         gBrowser.selectedBrowser.currentURI.spec,
         "mediaTab"
       );
diff --git a/browser/base/content/test/pageinfo/browser_pageinfo_permissions.js b/browser/base/content/test/pageinfo/browser_pageinfo_permissions.js
index ebf027811d..6b11ac19b9 100644
--- a/browser/base/content/test/pageinfo/browser_pageinfo_permissions.js
+++ b/browser/base/content/test/pageinfo/browser_pageinfo_permissions.js
@@ -7,8 +7,8 @@ const TEST_ORIGIN_CERT_ERROR = "https://expired.example.com";
 const LOW_TLS_VERSION = "https://tls1.example.com/";
 
 async function testPermissions(defaultPermission) {
-  await BrowserTestUtils.withNewTab(TEST_ORIGIN, async function (browser) {
-    let pageInfo = BrowserPageInfo(TEST_ORIGIN, "permTab");
+  await BrowserTestUtils.withNewTab(TEST_ORIGIN, async function () {
+    let pageInfo = BrowserCommands.pageInfo(TEST_ORIGIN, "permTab");
     await BrowserTestUtils.waitForEvent(pageInfo, "load");
 
     let defaultCheckbox = await TestUtils.waitForCondition(() =>
@@ -94,7 +94,7 @@ add_task(async function test_CertificateError() {
 
   await pageLoaded;
 
-  let pageInfo = BrowserPageInfo(TEST_ORIGIN_CERT_ERROR, "permTab");
+  let pageInfo = BrowserCommands.pageInfo(TEST_ORIGIN_CERT_ERROR, "permTab");
   await BrowserTestUtils.waitForEvent(pageInfo, "load");
   let permissionTab = pageInfo.document.getElementById("permTab");
   await TestUtils.waitForCondition(
@@ -145,7 +145,7 @@ add_task(async function test_NetworkError() {
 
   await pageLoaded;
 
-  let pageInfo = BrowserPageInfo(LOW_TLS_VERSION, "permTab");
+  let pageInfo = BrowserCommands.pageInfo(LOW_TLS_VERSION, "permTab");
   await BrowserTestUtils.waitForEvent(pageInfo, "load");
   let permissionTab = pageInfo.document.getElementById("permTab");
   await TestUtils.waitForCondition(
@@ -192,8 +192,8 @@ add_task(async function test_default_geo_permission() {
 
 // Test special behavior for cookie permissions.
 add_task(async function test_cookie_permission() {
-  await BrowserTestUtils.withNewTab(TEST_ORIGIN, async function (browser) {
-    let pageInfo = BrowserPageInfo(TEST_ORIGIN, "permTab");
+  await BrowserTestUtils.withNewTab(TEST_ORIGIN, async function () {
+    let pageInfo = BrowserCommands.pageInfo(TEST_ORIGIN, "permTab");
     await BrowserTestUtils.waitForEvent(pageInfo, "load");
 
     let defaultCheckbox = await TestUtils.waitForCondition(() =>
diff --git a/browser/base/content/test/pageinfo/browser_pageinfo_rtl.js b/browser/base/content/test/pageinfo/browser_pageinfo_rtl.js
index d0c06a03ff..677a58516e 100644
--- a/browser/base/content/test/pageinfo/browser_pageinfo_rtl.js
+++ b/browser/base/content/test/pageinfo/browser_pageinfo_rtl.js
@@ -1,18 +1,15 @@
 async function testPageInfo() {
-  await BrowserTestUtils.withNewTab(
-    "https://example.com",
-    async function (browser) {
-      let pageInfo = BrowserPageInfo();
-      await BrowserTestUtils.waitForEvent(pageInfo, "page-info-init");
-      is(
-        getComputedStyle(pageInfo.document.documentElement).direction,
-        "rtl",
-        "Should be RTL"
-      );
-      ok(true, "Didn't assert or crash");
-      pageInfo.close();
-    }
-  );
+  await BrowserTestUtils.withNewTab("https://example.com", async function () {
+    let pageInfo = BrowserCommands.pageInfo();
+    await BrowserTestUtils.waitForEvent(pageInfo, "page-info-init");
+    is(
+      getComputedStyle(pageInfo.document.documentElement).direction,
+      "rtl",
+      "Should be RTL"
+    );
+    ok(true, "Didn't assert or crash");
+    pageInfo.close();
+  });
 }
 
 add_task(async function test_page_info_rtl() {
diff --git a/browser/base/content/test/pageinfo/browser_pageinfo_security.js b/browser/base/content/test/pageinfo/browser_pageinfo_security.js
index 47df97db06..17ff0b9b75 100644
--- a/browser/base/content/test/pageinfo/browser_pageinfo_security.js
+++ b/browser/base/content/test/pageinfo/browser_pageinfo_security.js
@@ -24,7 +24,7 @@ add_task(async function test_ShowCertificate() {
     TEST_SUB_ORIGIN
   );
 
-  let pageInfo = BrowserPageInfo(TEST_SUB_ORIGIN, "securityTab");
+  let pageInfo = BrowserCommands.pageInfo(TEST_SUB_ORIGIN, "securityTab");
   await BrowserTestUtils.waitForEvent(pageInfo, "load");
   let pageInfoDoc = pageInfo.document;
   let securityTab = pageInfoDoc.getElementById("securityTab");
@@ -74,7 +74,7 @@ add_task(async function test_image() {
   let url = TEST_PATH + "moz.png";
   await BrowserTestUtils.openNewForegroundTab(gBrowser, url);
 
-  let pageInfo = BrowserPageInfo(url, "securityTab");
+  let pageInfo = BrowserCommands.pageInfo(url, "securityTab");
   await BrowserTestUtils.waitForEvent(pageInfo, "load");
   let pageInfoDoc = pageInfo.document;
   let securityTab = pageInfoDoc.getElementById("securityTab");
@@ -128,7 +128,10 @@ add_task(async function test_CertificateError() {
 
   await pageLoaded;
 
-  let pageInfo = BrowserPageInfo(TEST_ORIGIN_CERT_ERROR, "securityTab");
+  let pageInfo = BrowserCommands.pageInfo(
+    TEST_ORIGIN_CERT_ERROR,
+    "securityTab"
+  );
   await BrowserTestUtils.waitForEvent(pageInfo, "load");
   let pageInfoDoc = pageInfo.document;
   let securityTab = pageInfoDoc.getElementById("securityTab");
@@ -165,7 +168,7 @@ add_task(async function test_CertificateError() {
 add_task(async function test_SecurityHTTP() {
   await BrowserTestUtils.openNewForegroundTab(gBrowser, TEST_HTTP_ORIGIN);
 
-  let pageInfo = BrowserPageInfo(TEST_HTTP_ORIGIN, "securityTab");
+  let pageInfo = BrowserCommands.pageInfo(TEST_HTTP_ORIGIN, "securityTab");
   await BrowserTestUtils.waitForEvent(pageInfo, "load");
   let pageInfoDoc = pageInfo.document;
   let securityTab = pageInfoDoc.getElementById("securityTab");
@@ -201,7 +204,7 @@ add_task(async function test_SecurityHTTP() {
 add_task(async function test_ValidCert() {
   await BrowserTestUtils.openNewForegroundTab(gBrowser, TEST_ORIGIN);
 
-  let pageInfo = BrowserPageInfo(TEST_ORIGIN, "securityTab");
+  let pageInfo = BrowserCommands.pageInfo(TEST_ORIGIN, "securityTab");
   await BrowserTestUtils.waitForEvent(pageInfo, "load");
   let pageInfoDoc = pageInfo.document;
   let securityTab = pageInfoDoc.getElementById("securityTab");
@@ -237,11 +240,11 @@ add_task(async function test_ValidCert() {
 add_task(async function test_SiteData() {
   await SiteDataTestUtils.addToIndexedDB(TEST_ORIGIN);
 
-  await BrowserTestUtils.withNewTab(TEST_ORIGIN, async function (browser) {
+  await BrowserTestUtils.withNewTab(TEST_ORIGIN, async function () {
     let totalUsage = await SiteDataTestUtils.getQuotaUsage(TEST_ORIGIN);
     Assert.greater(totalUsage, 0, "The total usage should not be 0");
 
-    let pageInfo = BrowserPageInfo(TEST_ORIGIN, "securityTab");
+    let pageInfo = BrowserCommands.pageInfo(TEST_ORIGIN, "securityTab");
     await BrowserTestUtils.waitForEvent(pageInfo, "load");
     let pageInfoDoc = pageInfo.document;
 
@@ -302,8 +305,8 @@ add_task(async function test_Cookies() {
     value: "1",
   });
 
-  await BrowserTestUtils.withNewTab(TEST_ORIGIN, async function (browser) {
-    let pageInfo = BrowserPageInfo(TEST_ORIGIN, "securityTab");
+  await BrowserTestUtils.withNewTab(TEST_ORIGIN, async function () {
+    let pageInfo = BrowserCommands.pageInfo(TEST_ORIGIN, "securityTab");
     await BrowserTestUtils.waitForEvent(pageInfo, "load");
 
     let pageInfoDoc = pageInfo.document;
diff --git a/browser/base/content/test/pageinfo/browser_pageinfo_separate_private.js b/browser/base/content/test/pageinfo/browser_pageinfo_separate_private.js
index ac93b7ddb2..74289107a8 100644
--- a/browser/base/content/test/pageinfo/browser_pageinfo_separate_private.js
+++ b/browser/base/content/test/pageinfo/browser_pageinfo_separate_private.js
@@ -9,7 +9,7 @@ add_task(async function () {
     "https://example.com"
   );
   let browser = tab.linkedBrowser;
-  let pageInfo = BrowserPageInfo(browser.currentURI.spec);
+  let pageInfo = BrowserCommands.pageInfo(browser.currentURI.spec);
   await BrowserTestUtils.waitForEvent(pageInfo, "page-info-init");
   Assert.strictEqual(
     pageInfo.docShell.QueryInterface(Ci.nsILoadContext).usePrivateBrowsing,
@@ -25,7 +25,7 @@ add_task(async function () {
     "https://example.com"
   );
   let privateBrowser = privateTab.linkedBrowser;
-  let privatePageInfo = privateWindow.BrowserPageInfo(
+  let privatePageInfo = privateWindow.BrowserCommands.pageInfo(
     privateBrowser.currentURI.spec
   );
   await BrowserTestUtils.waitForEvent(privatePageInfo, "page-info-init");
diff --git a/browser/base/content/test/pageinfo/browser_pageinfo_svg_image.js b/browser/base/content/test/pageinfo/browser_pageinfo_svg_image.js
index 3934cd2aea..b00df72851 100644
--- a/browser/base/content/test/pageinfo/browser_pageinfo_svg_image.js
+++ b/browser/base/content/test/pageinfo/browser_pageinfo_svg_image.js
@@ -7,7 +7,7 @@ add_task(async function () {
   BrowserTestUtils.startLoadingURIString(gBrowser.selectedBrowser, URI);
   await BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser, false, URI);
 
-  const pageInfo = BrowserPageInfo(
+  const pageInfo = BrowserCommands.pageInfo(
     gBrowser.selectedBrowser.currentURI.spec,
     "mediaTab"
   );
diff --git a/browser/base/content/test/pageinfo/image.html b/browser/base/content/test/pageinfo/image.html
index 1261be8e7b..35e2d78e1e 100644
--- a/browser/base/content/test/pageinfo/image.html
+++ b/browser/base/content/test/pageinfo/image.html
@@ -1,5 +1,5 @@
 <html>
   <img src='moz.png' height=100 width=150 id='test-image'>
-  <video src='video.ogg' id='test-video'></video>
+  <video src='video.webm' id='test-video'></video>
   <audio src='audio.ogg' id='test-audio'></audio>
 </html>
diff --git a/browser/base/content/test/performance/StartupContentSubframe.sys.mjs b/browser/base/content/test/performance/StartupContentSubframe.sys.mjs
index a78e456afb..7d2d711765 100644
--- a/browser/base/content/test/performance/StartupContentSubframe.sys.mjs
+++ b/browser/base/content/test/performance/StartupContentSubframe.sys.mjs
@@ -16,7 +16,7 @@ export class StartupContentSubframeParent extends JSWindowActorParent {
 }
 
 export class StartupContentSubframeChild extends JSWindowActorChild {
-  async handleEvent(event) {
+  async handleEvent() {
     // When the remote subframe is loaded, an event will be fired to this actor,
     // which will cause us to send the `LoadedScripts` message to the parent
     // process.
diff --git a/browser/base/content/test/performance/browser_preferences_usage.js b/browser/base/content/test/performance/browser_preferences_usage.js
index 6bc623a360..b0ff140d19 100644
--- a/browser/base/content/test/performance/browser_preferences_usage.js
+++ b/browser/base/content/test/performance/browser_preferences_usage.js
@@ -107,7 +107,7 @@ add_task(async function startup() {
     "browser.startup.record": {
       // This pref is accessed in Nighly and debug builds only.
       min: 200,
-      max: 400,
+      max: 450,
     },
     "network.loadinfo.skip_type_assertion": {
       // This is accessed in debug only.
diff --git a/browser/base/content/test/performance/browser_startup_content.js b/browser/base/content/test/performance/browser_startup_content.js
index b0f861e47f..fac82ad990 100644
--- a/browser/base/content/test/performance/browser_startup_content.js
+++ b/browser/base/content/test/performance/browser_startup_content.js
@@ -57,18 +57,11 @@ const known_scripts = {
   ]),
 };
 
-if (!Services.appinfo.sessionHistoryInParent) {
-  known_scripts.modules.add(
-    "resource:///modules/sessionstore/ContentSessionStore.sys.mjs"
-  );
-}
-
 // Items on this list *might* load when creating the process, as opposed to
 // items in the main list, which we expect will always load.
 const intermittently_loaded_scripts = {
   modules: new Set([
     "resource://gre/modules/nsAsyncShutdown.sys.mjs",
-    "resource://gre/modules/sessionstore/Utils.sys.mjs",
 
     // Translations code which may be preffed on.
     "resource://gre/actors/TranslationsChild.sys.mjs",
@@ -119,7 +112,8 @@ add_task(async function () {
   let mm = gBrowser.selectedBrowser.messageManager;
   let promise = BrowserTestUtils.waitForMessage(mm, "Test:LoadedScripts");
 
-  // Load a custom frame script to avoid using ContentTask which loads Task.jsm
+  // Load a custom frame script to avoid using SpecialPowers.spawn which may
+  // load other modules.
   mm.loadFrameScript(
     "data:text/javascript,(" +
       function () {
diff --git a/browser/base/content/test/performance/browser_startup_mainthreadio.js b/browser/base/content/test/performance/browser_startup_mainthreadio.js
index b65ede26d5..a89a068f13 100644
--- a/browser/base/content/test/performance/browser_startup_mainthreadio.js
+++ b/browser/base/content/test/performance/browser_startup_mainthreadio.js
@@ -189,6 +189,7 @@ const startupPhases = {
     {
       // bug 1541601
       path: "PrfDef:channel-prefs.js",
+      condition: !MAC,
       stat: 1,
       read: 1,
       close: 1,
diff --git a/browser/base/content/test/performance/browser_tabdetach.js b/browser/base/content/test/performance/browser_tabdetach.js
index a860362f1f..3cbdde50fc 100644
--- a/browser/base/content/test/performance/browser_tabdetach.js
+++ b/browser/base/content/test/performance/browser_tabdetach.js
@@ -59,7 +59,7 @@ add_task(async function test_detach_not_overflowed() {
       expectedReflows: EXPECTED_REFLOWS,
       // we are opening a whole new window, so there's no point in tracking
       // rects being painted
-      frames: { filter: rects => [] },
+      frames: { filter: () => [] },
     }
   );
 
@@ -87,7 +87,7 @@ add_task(async function test_detach_overflowed() {
       expectedReflows: EXPECTED_REFLOWS,
       // we are opening a whole new window, so there's no point in tracking
       // rects being painted
-      frames: { filter: rects => [] },
+      frames: { filter: () => [] },
     }
   );
 
diff --git a/browser/base/content/test/performance/browser_tabopen.js b/browser/base/content/test/performance/browser_tabopen.js
index 2457812cb7..b7eabf4844 100644
--- a/browser/base/content/test/performance/browser_tabopen.js
+++ b/browser/base/content/test/performance/browser_tabopen.js
@@ -144,7 +144,7 @@ add_task(async function () {
   await withPerfObserver(
     async function () {
       let switchDone = BrowserTestUtils.waitForEvent(window, "TabSwitchDone");
-      BrowserOpenTab();
+      BrowserCommands.openTab();
       await BrowserTestUtils.waitForEvent(
         gBrowser.selectedTab,
         "TabAnimationEnd"
diff --git a/browser/base/content/test/performance/browser_tabopen_squeeze.js b/browser/base/content/test/performance/browser_tabopen_squeeze.js
index f92bdc2ea4..dd73f66030 100644
--- a/browser/base/content/test/performance/browser_tabopen_squeeze.js
+++ b/browser/base/content/test/performance/browser_tabopen_squeeze.js
@@ -52,7 +52,7 @@ add_task(async function () {
   await withPerfObserver(
     async function () {
       let switchDone = BrowserTestUtils.waitForEvent(window, "TabSwitchDone");
-      BrowserOpenTab();
+      BrowserCommands.openTab();
       await BrowserTestUtils.waitForEvent(
         gBrowser.selectedTab,
         "TabAnimationEnd"
diff --git a/browser/base/content/test/performance/browser_tabstrip_overflow_underflow.js b/browser/base/content/test/performance/browser_tabstrip_overflow_underflow.js
index 1fd33ed836..50d108c062 100644
--- a/browser/base/content/test/performance/browser_tabstrip_overflow_underflow.js
+++ b/browser/base/content/test/performance/browser_tabstrip_overflow_underflow.js
@@ -90,7 +90,7 @@ add_task(async function () {
   await withPerfObserver(
     async function () {
       let switchDone = BrowserTestUtils.waitForEvent(window, "TabSwitchDone");
-      BrowserOpenTab();
+      BrowserCommands.openTab();
       await BrowserTestUtils.waitForEvent(
         gBrowser.selectedTab,
         "TabAnimationEnd"
@@ -115,7 +115,7 @@ add_task(async function () {
   await withPerfObserver(
     async function () {
       let switchDone = BrowserTestUtils.waitForEvent(window, "TabSwitchDone");
-      BrowserOpenTab();
+      BrowserCommands.openTab();
       await switchDone;
       await TestUtils.waitForCondition(() => {
         return gBrowser.tabContainer.arrowScrollbox.hasAttribute(
diff --git a/browser/base/content/test/performance/browser_tabswitch.js b/browser/base/content/test/performance/browser_tabswitch.js
index bbbbac3a21..ba29efa662 100644
--- a/browser/base/content/test/performance/browser_tabswitch.js
+++ b/browser/base/content/test/performance/browser_tabswitch.js
@@ -59,6 +59,14 @@ add_task(async function () {
     getComputedStyle(gBrowser.selectedTab).paddingInlineStart
   );
   let minTabWidth = firstTabRect.width - 2 * tabPaddingStart;
+  if (AppConstants.platform == "macosx") {
+    // On macOS, after bug 1886729, gecko screenshots like the ones for this
+    // test can't screenshot the native titlebar. That, plus the fact that
+    // there's no border or shadow (see bug 1702653) means that we only end up
+    // with the tab text color changing, which is smaller than the tab
+    // background size.
+    minTabWidth = 0;
+  }
   let maxTabWidth = firstTabRect.width;
   let inRange = (val, min, max) => min <= val && val <= max;
 
@@ -84,11 +92,7 @@ add_task(async function () {
                   // The tab selection changes between 2 adjacent tabs, so we expect
                   // both to change color at once: this should be a single rect of the
                   // width of 2 tabs.
-                  inRange(
-                    r.w,
-                    minTabWidth - 1, // -1 for the border on Win7
-                    maxTabWidth * 2
-                  )
+                  inRange(r.w, minTabWidth, maxTabWidth * 2)
                 )
               )
           ),
diff --git a/browser/base/content/test/performance/browser_windowclose.js b/browser/base/content/test/performance/browser_windowclose.js
index 7d11779acc..11fa669be0 100644
--- a/browser/base/content/test/performance/browser_windowclose.js
+++ b/browser/base/content/test/performance/browser_windowclose.js
@@ -53,7 +53,7 @@ add_task(async function () {
     {
       expectedReflows: EXPECTED_REFLOWS,
       frames: {
-        filter(rects, frame, previousFrame) {
+        filter(rects, frame) {
           // Ignore the focus-out animation.
           if (isLikelyFocusChange(rects, frame)) {
             return [];
diff --git a/browser/base/content/test/performance/browser_windowopen.js b/browser/base/content/test/performance/browser_windowopen.js
index 02c6172948..b258cb67f5 100644
--- a/browser/base/content/test/performance/browser_windowopen.js
+++ b/browser/base/content/test/performance/browser_windowopen.js
@@ -44,7 +44,7 @@ add_task(async function () {
   let expectations = {
     expectedReflows: EXPECTED_REFLOWS,
     frames: {
-      filter(rects, frame, previousFrame) {
+      filter(rects, frame) {
         // The first screenshot we get in OSX / Windows shows an unfocused browser
         // window for some reason. See bug 1445161.
         if (!alreadyFocused && isLikelyFocusChange(rects, frame)) {
diff --git a/browser/base/content/test/performance/head.js b/browser/base/content/test/performance/head.js
index 29722e6bbe..42f7ae95fc 100644
--- a/browser/base/content/test/performance/head.js
+++ b/browser/base/content/test/performance/head.js
@@ -42,7 +42,7 @@ async function recordReflows(testPromise, win = window) {
   let reflows = [];
 
   let observer = {
-    reflow(start, end) {
+    reflow() {
       // Gather information about the current code path.
       reflows.push(new Error().stack);
 
@@ -50,7 +50,7 @@ async function recordReflows(testPromise, win = window) {
       dirtyFrame(win);
     },
 
-    reflowInterruptible(start, end) {
+    reflowInterruptible() {
       // Interruptible reflows are the reflows caused by the refresh
       // driver ticking. These are fine.
     },
@@ -99,11 +99,9 @@ async function recordReflows(testPromise, win = window) {
  *            // Sometimes, due to unpredictable timings, the reflow may be hit
  *            // less times.
  *            stack: [
- *              "select@chrome://global/content/bindings/textbox.xml",
- *              "focusAndSelectUrlBar@chrome://browser/content/browser.js",
- *              "openLinkIn@chrome://browser/content/utilityOverlay.js",
- *              "openUILinkIn@chrome://browser/content/utilityOverlay.js",
- *              "BrowserOpenTab@chrome://browser/content/browser.js",
+ *              "somefunction@chrome://somepackage/content/somefile.mjs",
+ *              "otherfunction@chrome://otherpackage/content/otherfile.js",
+ *              "morecode@resource://somewhereelse/SomeModule.sys.mjs",
  *            ],
  *            // We expect this particular reflow to happen up to 2 times.
  *            maxCount: 2,
@@ -113,10 +111,9 @@ async function recordReflows(testPromise, win = window) {
  *            // This reflow is caused by lorem ipsum. We expect this reflow
  *            // to only happen once, so we can omit the "maxCount" property.
  *            stack: [
- *              "get_scrollPosition@chrome://global/content/bindings/scrollbox.xml",
- *              "_fillTrailingGap@chrome://browser/content/tabbrowser.xml",
- *              "_handleNewTab@chrome://browser/content/tabbrowser.xml",
- *              "onxbltransitionend@chrome://browser/content/tabbrowser.xml",
+ *              "somefunction@chrome://somepackage/content/somefile.mjs",
+ *              "otherfunction@chrome://otherpackage/content/otherfile.js",
+ *              "morecode@resource://somewhereelse/SomeModule.sys.mjs",
  *            ],
  *          }
  *        ]
@@ -430,7 +427,7 @@ async function recordFrames(testPromise, win = window) {
 
   let frames = [];
 
-  let afterPaintListener = event => {
+  let afterPaintListener = () => {
     let width, height;
     canvas.width = width = win.innerWidth;
     canvas.height = height = win.innerHeight;
diff --git a/browser/base/content/test/permissions/browser_autoplay_blocked.js b/browser/base/content/test/permissions/browser_autoplay_blocked.js
index d81481d6a5..7fd45a4340 100644
--- a/browser/base/content/test/permissions/browser_autoplay_blocked.js
+++ b/browser/base/content/test/permissions/browser_autoplay_blocked.js
@@ -102,7 +102,7 @@ add_task(async function testMainViewVisible() {
 
   Services.prefs.setIntPref(AUTOPLAY_PREF, Ci.nsIAutoplay.BLOCKED);
 
-  await BrowserTestUtils.withNewTab(AUTOPLAY_PAGE, async function (browser) {
+  await BrowserTestUtils.withNewTab(AUTOPLAY_PAGE, async function () {
     let permissionsList = document.getElementById(
       "permission-popup-permission-list"
     );
diff --git a/browser/base/content/test/permissions/browser_canvas_fingerprinting_resistance.js b/browser/base/content/test/permissions/browser_canvas_fingerprinting_resistance.js
index dbb2d1ea32..62a49e359c 100644
--- a/browser/base/content/test/permissions/browser_canvas_fingerprinting_resistance.js
+++ b/browser/base/content/test/permissions/browser_canvas_fingerprinting_resistance.js
@@ -336,7 +336,7 @@ async function withNewTabInput(
   await SpecialPowers.spawn(browser, [], initTab);
   await enableResistFingerprinting(randomDataOnCanvasExtract, true);
   let popupShown = promisePopupShown();
-  await SpecialPowers.spawn(browser, [], function (host) {
+  await SpecialPowers.spawn(browser, [], function () {
     E10SUtils.wrapHandlingUserInput(content, true, function () {
       var button = content.document.getElementById("clickme");
       button.click();
@@ -361,11 +361,7 @@ async function withNewTabInput(
   await SpecialPowers.popPrefEnv();
 }
 
-async function doTestInput(
-  randomDataOnCanvasExtract,
-  grantPermission,
-  autoDeclineNoInput
-) {
+async function doTestInput(randomDataOnCanvasExtract, grantPermission) {
   await BrowserTestUtils.withNewTab(
     kUrl,
     withNewTabInput.bind(null, randomDataOnCanvasExtract, grantPermission)
diff --git a/browser/base/content/test/permissions/browser_site_scoped_permissions.js b/browser/base/content/test/permissions/browser_site_scoped_permissions.js
index 7a8953de47..949d7a0596 100644
--- a/browser/base/content/test/permissions/browser_site_scoped_permissions.js
+++ b/browser/base/content/test/permissions/browser_site_scoped_permissions.js
@@ -21,7 +21,7 @@ add_task(async function testSiteScopedPermissionSubdomainAffectsBaseDomain() {
     );
   let id = "3rdPartyStorage^https://example.org";
 
-  await BrowserTestUtils.withNewTab(EMPTY_PAGE, async function (browser) {
+  await BrowserTestUtils.withNewTab(EMPTY_PAGE, async function () {
     Services.perms.addFromPrincipal(
       subdomainPrincipal,
       id,
@@ -76,49 +76,46 @@ add_task(async function testSiteScopedPermissionBaseDomainAffectsSubdomain() {
     Services.scriptSecurityManager.createContentPrincipalFromOrigin(origin);
   let id = "3rdPartyStorage^https://example.org";
 
-  await BrowserTestUtils.withNewTab(
-    SUBDOMAIN_EMPTY_PAGE,
-    async function (browser) {
-      Services.perms.addFromPrincipal(principal, id, SitePermissions.ALLOW);
-      await openPermissionPopup();
-
-      let permissionsList = document.getElementById(
-        "permission-popup-permission-list"
-      );
-      let listEntryCount = permissionsList.querySelectorAll(
-        ".permission-popup-permission-item"
-      ).length;
-      is(
-        listEntryCount,
-        1,
-        "Permission exists on base domain when set on subdomain"
-      );
-
-      closePermissionPopup();
-
-      Services.perms.removeFromPrincipal(principal, id);
-
-      // We intentionally turn off a11y_checks, because the following function
-      // is expected to click a toolbar button that may be already hidden
-      // with "display:none;". The permissions panel anchor is hidden because
-      // the last permission was removed, however we force opening the panel
-      // anyways in order to test that the list has been properly emptied:
-      AccessibilityUtils.setEnv({
-        mustHaveAccessibleRule: false,
-      });
-      await openPermissionPopup();
-      AccessibilityUtils.resetEnv();
-
-      listEntryCount = permissionsList.querySelectorAll(
-        ".permission-popup-permission-item-3rdPartyStorage"
-      ).length;
-      is(
-        listEntryCount,
-        0,
-        "Permission removed on base domain when removed on subdomain"
-      );
-
-      await closePermissionPopup();
-    }
-  );
+  await BrowserTestUtils.withNewTab(SUBDOMAIN_EMPTY_PAGE, async function () {
+    Services.perms.addFromPrincipal(principal, id, SitePermissions.ALLOW);
+    await openPermissionPopup();
+
+    let permissionsList = document.getElementById(
+      "permission-popup-permission-list"
+    );
+    let listEntryCount = permissionsList.querySelectorAll(
+      ".permission-popup-permission-item"
+    ).length;
+    is(
+      listEntryCount,
+      1,
+      "Permission exists on base domain when set on subdomain"
+    );
+
+    closePermissionPopup();
+
+    Services.perms.removeFromPrincipal(principal, id);
+
+    // We intentionally turn off a11y_checks, because the following function
+    // is expected to click a toolbar button that may be already hidden
+    // with "display:none;". The permissions panel anchor is hidden because
+    // the last permission was removed, however we force opening the panel
+    // anyways in order to test that the list has been properly emptied:
+    AccessibilityUtils.setEnv({
+      mustHaveAccessibleRule: false,
+    });
+    await openPermissionPopup();
+    AccessibilityUtils.resetEnv();
+
+    listEntryCount = permissionsList.querySelectorAll(
+      ".permission-popup-permission-item-3rdPartyStorage"
+    ).length;
+    is(
+      listEntryCount,
+      0,
+      "Permission removed on base domain when removed on subdomain"
+    );
+
+    await closePermissionPopup();
+  });
 });
diff --git a/browser/base/content/test/permissions/browser_temporary_permissions_navigation.js b/browser/base/content/test/permissions/browser_temporary_permissions_navigation.js
index 7da79b1810..490da04374 100644
--- a/browser/base/content/test/permissions/browser_temporary_permissions_navigation.js
+++ b/browser/base/content/test/permissions/browser_temporary_permissions_navigation.js
@@ -41,7 +41,7 @@ add_task(async function testTempPermissionOnReload() {
     reloaded = BrowserTestUtils.browserLoaded(browser, false, origin);
 
     // Reload as a user (should remove the temp permission).
-    BrowserReload();
+    BrowserCommands.reload();
 
     await reloaded;
 
diff --git a/browser/base/content/test/plugins/head.js b/browser/base/content/test/plugins/head.js
index 4f6c25b92a..76f87dfc43 100644
--- a/browser/base/content/test/plugins/head.js
+++ b/browser/base/content/test/plugins/head.js
@@ -147,7 +147,7 @@ function promiseWaitForFocus(aWindow) {
  * @return Promise
  */
 function waitForNotificationBar(notificationID, browser, callback) {
-  return new Promise((resolve, reject) => {
+  return new Promise(resolve => {
     let notification;
     let notificationBox = gBrowser.getNotificationBox(browser);
     waitForCondition(
@@ -189,7 +189,7 @@ function waitForNotificationShown(notification, callback) {
   }
   PopupNotifications.panel.addEventListener(
     "popupshown",
-    function (e) {
+    function () {
       callback();
     },
     { once: true }
diff --git a/browser/base/content/test/popupNotifications/browser_popupNotification.js b/browser/base/content/test/popupNotifications/browser_popupNotification.js
index 235aa90b5f..4479cb1ee7 100644
--- a/browser/base/content/test/popupNotifications/browser_popupNotification.js
+++ b/browser/base/content/test/popupNotifications/browser_popupNotification.js
@@ -26,7 +26,7 @@ var tests = [
       checkPopup(popup, this.notifyObj);
       triggerMainCommand(popup);
     },
-    onHidden(popup) {
+    onHidden() {
       ok(this.notifyObj.mainActionClicked, "mainAction was clicked");
       ok(
         !this.notifyObj.dismissalCallbackTriggered,
@@ -55,7 +55,7 @@ var tests = [
       checkPopup(popup, this.notifyObj);
       triggerSecondaryCommand(popup, 0);
     },
-    onHidden(popup) {
+    onHidden() {
       ok(this.notifyObj.secondaryActionClicked, "secondaryAction was clicked");
       ok(
         !this.notifyObj.dismissalCallbackTriggered,
@@ -89,7 +89,7 @@ var tests = [
       checkPopup(popup, this.notifyObj);
       triggerSecondaryCommand(popup, 1);
     },
-    onHidden(popup) {
+    onHidden() {
       ok(
         this.extraSecondaryActionClicked,
         "extra secondary action was clicked"
@@ -123,7 +123,7 @@ var tests = [
       checkPopup(popup, this.notifyObj);
       triggerSecondaryCommand(popup, 2);
     },
-    onHidden(popup) {
+    onHidden() {
       ok(
         this.extraSecondaryActionClicked,
         "extra secondary action was clicked"
@@ -145,7 +145,7 @@ var tests = [
       checkPopup(popup, this.notifyObj);
       dismissNotification(popup);
     },
-    onHidden(popup) {
+    onHidden() {
       ok(
         this.notifyObj.dismissalCallbackTriggered,
         "dismissal callback triggered"
@@ -205,7 +205,7 @@ var tests = [
       // switch back to the old browser
       gBrowser.selectedTab = this.oldSelectedTab;
     },
-    onHidden(popup) {
+    onHidden() {
       // actually remove the notification to prevent it from reappearing
       ok(
         wrongBrowserNotificationObject.dismissalCallbackTriggered,
@@ -247,7 +247,7 @@ var tests = [
       checkPopup(popup, this.notifyObj);
       this.notification2.remove();
     },
-    onHidden(popup) {
+    onHidden() {
       ok(
         !this.notifyObj.dismissalCallbackTriggered,
         "dismissal callback wasn't triggered"
@@ -276,7 +276,7 @@ var tests = [
       is(popup.children.length, 1, "only one notification left");
       triggerSecondaryCommand(popup, 0);
     },
-    onHidden(popup) {
+    onHidden() {
       ok(this.testNotif1.mainActionClicked, "main action #1 was clicked");
       ok(
         !this.testNotif1.secondaryActionClicked,
@@ -316,7 +316,7 @@ var tests = [
       );
       triggerMainCommand(popup);
     },
-    onHidden(popup) {
+    onHidden() {
       ok(!this.notifyObj.mainActionClicked, "mainAction was not clicked");
       ok(
         !this.notifyObj.dismissalCallbackTriggered,
@@ -348,7 +348,7 @@ var tests = [
       );
       triggerMainCommand(popup);
     },
-    onHidden(popup) {
+    onHidden() {
       ok(!this.notifyObj.mainActionClicked, "mainAction was not clicked");
       ok(
         !this.notifyObj.dismissalCallbackTriggered,
@@ -380,7 +380,7 @@ var tests = [
       );
       dismissNotification(popup);
     },
-    onHidden(popup) {
+    onHidden() {
       // Remove the notifications
       this.firstNotification.remove();
       this.secondNotification.remove();
diff --git a/browser/base/content/test/popupNotifications/browser_popupNotification_2.js b/browser/base/content/test/popupNotifications/browser_popupNotification_2.js
index 8738a3b605..1b0dea66f6 100644
--- a/browser/base/content/test/popupNotifications/browser_popupNotification_2.js
+++ b/browser/base/content/test/popupNotifications/browser_popupNotification_2.js
@@ -24,7 +24,7 @@ var tests = [
       checkPopup(popup, this.notifyObj);
       dismissNotification(popup);
     },
-    onHidden(popup) {
+    onHidden() {
       ok(
         this.notifyObj.dismissalCallbackTriggered,
         "dismissal callback triggered"
@@ -52,7 +52,7 @@ var tests = [
       );
       dismissNotification(popup);
     },
-    onHidden(popup) {
+    onHidden() {
       let icon = document.getElementById("geo-notification-icon");
       isnot(
         icon.getBoundingClientRect().width,
@@ -84,7 +84,7 @@ var tests = [
       });
       this.notification = showNotification(this.notifyObj);
     },
-    async onShown(popup) {
+    async onShown() {
       this.complete = false;
       // eslint-disable-next-line @microsoft/sdl/no-insecure-url
       await promiseTabLoadEvent(gBrowser.selectedTab, "http://example.org/");
@@ -95,7 +95,7 @@ var tests = [
       // eslint-disable-next-line @microsoft/sdl/no-insecure-url
       await promiseTabLoadEvent(gBrowser.selectedTab, "http://example.org/");
     },
-    onHidden(popup) {
+    onHidden() {
       ok(
         this.complete,
         "Should only have hidden the notification after 3 page loads"
@@ -122,7 +122,7 @@ var tests = [
       });
       this.notification = showNotification(this.notifyObj);
     },
-    async onShown(popup) {
+    async onShown() {
       this.complete = false;
       // eslint-disable-next-line @microsoft/sdl/no-insecure-url
       await promiseTabLoadEvent(gBrowser.selectedTab, "http://example.org/");
@@ -134,7 +134,7 @@ var tests = [
       // eslint-disable-next-line @microsoft/sdl/no-insecure-url
       await promiseTabLoadEvent(gBrowser.selectedTab, "http://example.org/");
     },
-    onHidden(popup) {
+    onHidden() {
       ok(
         this.complete,
         "Should only have hidden the notification after the timeout was passed"
@@ -172,7 +172,7 @@ var tests = [
       this.complete = true;
       dismissNotification(popup);
     },
-    onHidden(popup) {
+    onHidden() {
       ok(
         this.complete,
         "Should only have hidden the notification after it was dismissed"
@@ -212,7 +212,7 @@ var tests = [
       checkPopup(popup, this.notifyObj);
       dismissNotification(popup);
     },
-    onHidden(popup) {
+    onHidden() {
       this.notification.remove();
       this.box.remove();
     },
@@ -272,7 +272,7 @@ var tests = [
       let notification = popup.children[0];
       EventUtils.synthesizeMouseAtCenter(notification.closebutton, {});
     },
-    onHidden(popup) {
+    onHidden() {
       ok(
         this.notifyObj.dismissalCallbackTriggered,
         "dismissal callback triggered"
@@ -302,7 +302,7 @@ var tests = [
       );
       dismissNotification(popup);
     },
-    onHidden(popup) {
+    onHidden() {
       ok(
         this.notifyObj.dismissalCallbackTriggered,
         "dismissal callback triggered"
diff --git a/browser/base/content/test/popupNotifications/browser_popupNotification_3.js b/browser/base/content/test/popupNotifications/browser_popupNotification_3.js
index 1b7626c660..1d8b6b473b 100644
--- a/browser/base/content/test/popupNotifications/browser_popupNotification_3.js
+++ b/browser/base/content/test/popupNotifications/browser_popupNotification_3.js
@@ -27,7 +27,7 @@ var tests = [
       checkPopup(popup, this.notifyObj);
       dismissNotification(popup);
     },
-    onHidden(popup) {
+    onHidden() {
       ok(
         !this.notifyObj.dismissalCallbackTriggered,
         "dismissal callback wasn't triggered"
@@ -70,7 +70,7 @@ var tests = [
 
       dismissNotification(popup);
     },
-    onHidden(popup) {
+    onHidden() {
       this.notification1.remove();
       ok(
         this.notifyObj1.removedCallbackTriggered,
@@ -127,7 +127,7 @@ var tests = [
 
       dismissNotification(popup);
     },
-    onHidden(popup) {
+    onHidden() {
       this.notificationNew.remove();
       gBrowser.removeTab(gBrowser.selectedTab);
 
@@ -156,7 +156,7 @@ var tests = [
         dismissNotification(popup);
       });
     },
-    onHidden(popup) {
+    onHidden() {
       ok(
         !this.notifyObj.mainActionClicked,
         "mainAction was not clicked because it was too soon"
@@ -188,7 +188,7 @@ var tests = [
         triggerMainCommand(popup);
       }, 500);
     },
-    onHidden(popup) {
+    onHidden() {
       ok(
         this.notifyObj.mainActionClicked,
         "mainAction was clicked after the delay"
@@ -308,7 +308,7 @@ var tests = [
       };
       showNotification(this.notifyObj);
     },
-    async onShown(popup) {
+    async onShown() {
       info("Adding observer and performing navigation");
 
       await Promise.all([
diff --git a/browser/base/content/test/popupNotifications/browser_popupNotification_4.js b/browser/base/content/test/popupNotifications/browser_popupNotification_4.js
index b0e8f016ef..3ea0e943a3 100644
--- a/browser/base/content/test/popupNotifications/browser_popupNotification_4.js
+++ b/browser/base/content/test/popupNotifications/browser_popupNotification_4.js
@@ -23,7 +23,7 @@ var tests = [
       checkPopup(popup, this.testNotif);
       triggerMainCommand(popup);
     },
-    onHidden(popup) {
+    onHidden() {
       ok(this.testNotif.mainActionClicked, "main action has been triggered");
     },
   },
@@ -38,7 +38,7 @@ var tests = [
       checkPopup(popup, this.testNotif);
       triggerSecondaryCommand(popup, 0);
     },
-    onHidden(popup) {
+    onHidden() {
       ok(
         this.testNotif.secondaryActionClicked,
         "secondary action has been triggered"
@@ -83,7 +83,7 @@ var tests = [
 
       dismissNotification(popup);
     },
-    onHidden(popup) {
+    onHidden() {
       this.notification1.remove();
       this.notification2.remove();
     },
@@ -213,7 +213,7 @@ var tests = [
       checkPopup(popup, this.notifyObj);
       triggerMainCommand(popup);
     },
-    onHidden(popup) {
+    onHidden() {
       ok(
         this.notifyObj.dismissalCallbackTriggered,
         "dismissal callback was triggered"
@@ -237,7 +237,7 @@ var tests = [
       checkPopup(popup, this.notifyObj);
       triggerSecondaryCommand(popup, 0);
     },
-    onHidden(popup) {
+    onHidden() {
       ok(
         this.notifyObj.dismissalCallbackTriggered,
         "dismissal callback was triggered"
diff --git a/browser/base/content/test/popupNotifications/browser_popupNotification_5.js b/browser/base/content/test/popupNotifications/browser_popupNotification_5.js
index 0ec5de0c3a..48640b9b00 100644
--- a/browser/base/content/test/popupNotifications/browser_popupNotification_5.js
+++ b/browser/base/content/test/popupNotifications/browser_popupNotification_5.js
@@ -62,7 +62,7 @@ var tests = [
       this.notification1.remove();
       this.notification2.remove();
     },
-    onHidden(popup) {},
+    onHidden() {},
   },
   // The anchor icon should be shown for notifications in background windows.
   {
@@ -116,7 +116,7 @@ var tests = [
       this.complete = true;
       triggerSecondaryCommand(popup, 0);
     },
-    onHidden(popup) {
+    onHidden() {
       ok(
         !this.complete,
         "Should have hidden the notification after navigation"
@@ -155,7 +155,7 @@ var tests = [
       this.complete = true;
       triggerSecondaryCommand(popup, 0);
     },
-    onHidden(popup) {
+    onHidden() {
       ok(
         this.complete,
         "Should have hidden the notification after clicking Not Now"
@@ -174,7 +174,7 @@ var tests = [
       this.notifyObj.options.persistent = true;
       gNotification = showNotification(this.notifyObj);
     },
-    async onShown(popup) {
+    async onShown() {
       this.oldSelectedTab = gBrowser.selectedTab;
       await BrowserTestUtils.openNewForegroundTab(
         gBrowser,
@@ -182,7 +182,7 @@ var tests = [
         "http://example.com/"
       );
     },
-    onHidden(popup) {
+    onHidden() {
       ok(true, "Should have hidden the notification after tab switch");
       gBrowser.removeTab(gBrowser.selectedTab);
       gBrowser.selectedTab = this.oldSelectedTab;
@@ -318,7 +318,7 @@ var tests = [
       this.notification1.remove();
       this.notification2.remove();
     },
-    onHidden(popup) {},
+    onHidden() {},
   },
   // Test that persistent notifications are shown stacked by anchor on update
   {
@@ -363,7 +363,7 @@ var tests = [
       this.notification2.remove();
       this.notification3.remove();
     },
-    onHidden(popup) {},
+    onHidden() {},
   },
   // Test that on closebutton click, only the persistent notification
   // that contained the closebutton loses its persistent status.
diff --git a/browser/base/content/test/popupNotifications/browser_popupNotification_accesskey.js b/browser/base/content/test/popupNotifications/browser_popupNotification_accesskey.js
index 4a68105e27..7e8e3f0269 100644
--- a/browser/base/content/test/popupNotifications/browser_popupNotification_accesskey.js
+++ b/browser/base/content/test/popupNotifications/browser_popupNotification_accesskey.js
@@ -36,7 +36,7 @@ var tests = [
       // process of being hidden right now.
       isnot(popup.state, "hiding", "popup is not hiding");
     },
-    onHidden(popup) {
+    onHidden() {
       window.removeEventListener("command", commandTriggered, true);
       ok(buttonPressed, "button pressed");
     },
diff --git a/browser/base/content/test/popupNotifications/browser_popupNotification_keyboard.js b/browser/base/content/test/popupNotifications/browser_popupNotification_keyboard.js
index 5c20751c3f..bd6fd38d3f 100644
--- a/browser/base/content/test/popupNotifications/browser_popupNotification_keyboard.js
+++ b/browser/base/content/test/popupNotifications/browser_popupNotification_keyboard.js
@@ -46,7 +46,7 @@ var tests = [
       checkPopup(popup, this.notifyObj);
       EventUtils.synthesizeKey("KEY_Escape");
     },
-    onHidden(popup) {
+    onHidden() {
       ok(!this.notifyObj.mainActionClicked, "mainAction was not clicked");
       ok(this.notifyObj.secondaryActionClicked, "secondaryAction was clicked");
       ok(
@@ -77,7 +77,7 @@ var tests = [
       checkPopup(popup, this.notifyObj);
       EventUtils.synthesizeKey("KEY_Escape");
     },
-    onHidden(popup) {
+    onHidden() {
       ok(!this.notifyObj.mainActionClicked, "mainAction was not clicked");
       ok(
         !this.notifyObj.secondaryActionClicked,
@@ -123,7 +123,7 @@ var tests = [
       is(document.activeElement, popup.children[0].closebutton);
       this.notification.remove();
     },
-    onHidden(popup) {},
+    onHidden() {},
   },
   // Test that you can switch between active notifications with the space key
   // and that the notification is focused on selection.
diff --git a/browser/base/content/test/popupNotifications/browser_popupNotification_no_anchors.js b/browser/base/content/test/popupNotifications/browser_popupNotification_no_anchors.js
index a73e1f5948..68e782cea6 100644
--- a/browser/base/content/test/popupNotifications/browser_popupNotification_no_anchors.js
+++ b/browser/base/content/test/popupNotifications/browser_popupNotification_no_anchors.js
@@ -43,7 +43,7 @@ var tests = [
       );
       dismissNotification(popup);
     },
-    onHidden(popup) {
+    onHidden() {
       this.notification.remove();
       gBrowser.removeTab(gBrowser.selectedTab);
       gBrowser.selectedTab = this.oldSelectedTab;
@@ -85,7 +85,7 @@ var tests = [
       );
       dismissNotification(popup);
     },
-    onHidden(popup) {
+    onHidden() {
       this.notification.remove();
       gBrowser.removeTab(gBrowser.selectedTab);
       gBrowser.selectedTab = this.oldSelectedTab;
@@ -135,7 +135,7 @@ var tests = [
       checkPopup(popup, this.notifyObj);
       dismissNotification(popup);
     },
-    onHidden(popup) {
+    onHidden() {
       this.notification.remove();
       gBrowser.removeTab(gBrowser.selectedTab);
       gBrowser.selectedTab = this.oldSelectedTab;
diff --git a/browser/base/content/test/popupNotifications/browser_popupNotification_security_delay.js b/browser/base/content/test/popupNotifications/browser_popupNotification_security_delay.js
index 3b027bc1ef..140c2be2fd 100644
--- a/browser/base/content/test/popupNotifications/browser_popupNotification_security_delay.js
+++ b/browser/base/content/test/popupNotifications/browser_popupNotification_security_delay.js
@@ -227,7 +227,7 @@ add_task(async function test_notificationReshowTabSwitch() {
   let panelShownPromise;
 
   info("Open a new tab which hides the notification panel.");
-  await BrowserTestUtils.withNewTab("https://example.com", async browser => {
+  await BrowserTestUtils.withNewTab("https://example.com", async () => {
     info("Wait for panel to be hidden by tab switch.");
     await panelHiddenPromise;
     info(
diff --git a/browser/base/content/test/popups/browser_popup_close_main_window.js b/browser/base/content/test/popups/browser_popup_close_main_window.js
index 148e937bca..abf6c43c3f 100644
--- a/browser/base/content/test/popups/browser_popup_close_main_window.js
+++ b/browser/base/content/test/popups/browser_popup_close_main_window.js
@@ -37,7 +37,7 @@ add_task(async function closing_last_window_equals_quitting() {
   Services.obs.addObserver(obs, "browser-lastwindow-close-requested");
   let newWin = await BrowserTestUtils.openNewBrowserWindow();
   let closedPromise = BrowserTestUtils.windowClosed(newWin);
-  newWin.BrowserTryToCloseWindow();
+  newWin.BrowserCommands.tryToCloseWindow();
   await closedPromise;
   is(observed, 1, "Got a notification for closing the normal window.");
   Services.obs.removeObserver(obs, "browser-lastwindow-close-requested");
@@ -68,12 +68,12 @@ add_task(async function closing_last_window_equals_quitting() {
   });
   let popupWin = await popupPromise;
   let closedPromise = BrowserTestUtils.windowClosed(newWin);
-  newWin.BrowserTryToCloseWindow();
+  newWin.BrowserCommands.tryToCloseWindow();
   await closedPromise;
   is(observed, 0, "Got no notification for closing the normal window.");
 
   closedPromise = BrowserTestUtils.windowClosed(popupWin);
-  popupWin.BrowserTryToCloseWindow();
+  popupWin.BrowserCommands.tryToCloseWindow();
   await closedPromise;
   is(
     observed,
diff --git a/browser/base/content/test/protectionsUI/browser_protectionsUI.js b/browser/base/content/test/protectionsUI/browser_protectionsUI.js
index 5dc6acebf7..e512f7a415 100644
--- a/browser/base/content/test/protectionsUI/browser_protectionsUI.js
+++ b/browser/base/content/test/protectionsUI/browser_protectionsUI.js
@@ -500,7 +500,7 @@ add_task(async function testNumberOfBlockedTrackers() {
   // attribute will only be set if the previous counter is zero. Instead, we
   // wait for the change of the text content of the counter.
   let updateCounterPromise = new Promise(resolve => {
-    let mut = new MutationObserver(mutations => {
+    let mut = new MutationObserver(() => {
       resolve();
       mut.disconnect();
     });
diff --git a/browser/base/content/test/protectionsUI/browser_protectionsUI_cookies_subview.js b/browser/base/content/test/protectionsUI/browser_protectionsUI_cookies_subview.js
index 00281ac415..1346fb94c1 100644
--- a/browser/base/content/test/protectionsUI/browser_protectionsUI_cookies_subview.js
+++ b/browser/base/content/test/protectionsUI/browser_protectionsUI_cookies_subview.js
@@ -397,7 +397,7 @@ add_task(async function testCookiesSubViewAllowedHeuristic() {
   let popup;
   let windowCreated = TestUtils.topicObserved(
     "chrome-document-global-created",
-    (subject, data) => {
+    subject => {
       popup = subject;
       return true;
     }
diff --git a/browser/base/content/test/protectionsUI/browser_protectionsUI_fetch.js b/browser/base/content/test/protectionsUI/browser_protectionsUI_fetch.js
index 26b131d4eb..02aa21474d 100644
--- a/browser/base/content/test/protectionsUI/browser_protectionsUI_fetch.js
+++ b/browser/base/content/test/protectionsUI/browser_protectionsUI_fetch.js
@@ -15,7 +15,7 @@ add_task(async function test_fetch() {
       await SpecialPowers.spawn(newTabBrowser, [], async function () {
         await content.wrappedJSObject
           .test_fetch()
-          .then(response => Assert.ok(false, "should have denied the request"))
+          .then(() => Assert.ok(false, "should have denied the request"))
           .catch(e => Assert.ok(true, `Caught exception: ${e}`));
       });
       await contentBlockingEvent;
diff --git a/browser/base/content/test/protectionsUI/browser_protectionsUI_info_message.js b/browser/base/content/test/protectionsUI/browser_protectionsUI_info_message.js
index fadfaaab98..1e07db2689 100644
--- a/browser/base/content/test/protectionsUI/browser_protectionsUI_info_message.js
+++ b/browser/base/content/test/protectionsUI/browser_protectionsUI_info_message.js
@@ -51,10 +51,10 @@ add_task(async function testPanelInfoMessage() {
   });
 
   // Test that the info message is displayed when the panel opens
-  let container = document.getElementById("messaging-system-message-container");
+  let container = document.getElementById("info-message-container");
   let message = document.getElementById("protections-popup-message");
   let learnMoreLink = document.querySelector(
-    "#messaging-system-message-container .text-link"
+    "#info-message-container .text-link"
   );
 
   // Check the visibility of the info message.
diff --git a/browser/base/content/test/referrer/head.js b/browser/base/content/test/referrer/head.js
index c812d73e80..34a5f2a58e 100644
--- a/browser/base/content/test/referrer/head.js
+++ b/browser/base/content/test/referrer/head.js
@@ -165,7 +165,7 @@ function delayedStartupFinished(aWindow) {
  * @return {Promise}
  * @resolves With the tab once it's loaded.
  */
-function someTabLoaded(aWindow) {
+function someTabLoaded() {
   return BrowserTestUtils.waitForNewTab(gTestWindow.gBrowser, null, true);
 }
 
diff --git a/browser/base/content/test/sanitize/browser_cookiePermission_aboutURL.js b/browser/base/content/test/sanitize/browser_cookiePermission_aboutURL.js
index ada8286437..ff6badb535 100644
--- a/browser/base/content/test/sanitize/browser_cookiePermission_aboutURL.js
+++ b/browser/base/content/test/sanitize/browser_cookiePermission_aboutURL.js
@@ -15,10 +15,10 @@ function checkDataForAboutURL() {
       {}
     );
     let request = indexedDB.openForPrincipal(principal, "TestDatabase", 1);
-    request.onupgradeneeded = function (e) {
+    request.onupgradeneeded = function () {
       data = false;
     };
-    request.onsuccess = function (e) {
+    request.onsuccess = function () {
       resolve(data);
     };
   });
diff --git a/browser/base/content/test/sanitize/browser_sanitize-timespans.js b/browser/base/content/test/sanitize/browser_sanitize-timespans.js
index f9be12775b..28b528d71f 100644
--- a/browser/base/content/test/sanitize/browser_sanitize-timespans.js
+++ b/browser/base/content/test/sanitize/browser_sanitize-timespans.js
@@ -20,7 +20,7 @@ function promiseFormHistoryRemoved() {
 function promiseDownloadRemoved(list) {
   return new Promise(resolve => {
     let view = {
-      onDownloadRemoved(download) {
+      onDownloadRemoved() {
         list.removeView(view);
         resolve();
       },
diff --git a/browser/base/content/test/sanitize/browser_sanitize-timespans_v2.js b/browser/base/content/test/sanitize/browser_sanitize-timespans_v2.js
index c732262a1a..067a651890 100644
--- a/browser/base/content/test/sanitize/browser_sanitize-timespans_v2.js
+++ b/browser/base/content/test/sanitize/browser_sanitize-timespans_v2.js
@@ -23,7 +23,7 @@ function promiseFormHistoryRemoved() {
 function promiseDownloadRemoved(list) {
   return new Promise(resolve => {
     let view = {
-      onDownloadRemoved(download) {
+      onDownloadRemoved() {
         list.removeView(view);
         resolve();
       },
diff --git a/browser/base/content/test/sanitize/browser_sanitizeDialog_v2.js b/browser/base/content/test/sanitize/browser_sanitizeDialog_v2.js
index 8ae0263c82..ecdc5490d4 100644
--- a/browser/base/content/test/sanitize/browser_sanitizeDialog_v2.js
+++ b/browser/base/content/test/sanitize/browser_sanitizeDialog_v2.js
@@ -844,14 +844,14 @@ add_task(async function testLoadtimeTelemetry() {
   let loadTimeDistribution = Glean.privacySanitize.loadTime.testGetValue();
 
   let expectedNumberOfCounts = Object.entries(EXPECTED_CONTEXT_COUNTS).reduce(
-    (acc, [key, value]) => acc + value,
+    (acc, [, value]) => acc + value,
     0
   );
   // No guarantees from timers means no guarantees on buckets.
   // But we can guarantee it's only two samples.
   is(
     Object.entries(loadTimeDistribution.values).reduce(
-      (acc, [bucket, count]) => acc + count,
+      (acc, [, count]) => acc + count,
       0
     ),
     expectedNumberOfCounts,
diff --git a/browser/base/content/test/sanitize/browser_sanitizeDialog_v2_dataSizes.js b/browser/base/content/test/sanitize/browser_sanitizeDialog_v2_dataSizes.js
index ccb3c7d519..736df32e81 100644
--- a/browser/base/content/test/sanitize/browser_sanitizeDialog_v2_dataSizes.js
+++ b/browser/base/content/test/sanitize/browser_sanitizeDialog_v2_dataSizes.js
@@ -10,6 +10,14 @@ ChromeUtils.defineESModuleGetters(this, {
   Sanitizer: "resource:///modules/Sanitizer.sys.mjs",
 });
 
+const LARGE_USAGE_NUM = 100000000000000000000000000000000000000000000000000;
+
+function isIframeOverflowing(win) {
+  return (
+    win.scrollWidth > win.clientWidth || win.scrollHeight > win.clientHeight
+  );
+}
+
 add_setup(async function () {
   await blankSlate();
   registerCleanupFunction(async function () {
@@ -275,7 +283,7 @@ add_task(async function testClearingBeforeDataSizesLoad() {
       info("stub called");
 
       info("This promise should never resolve");
-      await new Promise(resolve => {});
+      await new Promise(() => {});
     });
   dh.onload = async function () {
     // we don't need to initiate a event listener to wait for the resolver to be assigned for this
@@ -308,3 +316,58 @@ add_task(async function testClearingBeforeDataSizesLoad() {
   // Restore the sandbox after the test is complete
   sandbox.restore();
 });
+
+// tests the dialog resizing upon wrapping of text
+// so that the clear buttons do not get cut out of the dialog.
+add_task(async function testPossibleWrappingOfDialog() {
+  await blankSlate();
+
+  let dh = new ClearHistoryDialogHelper({
+    checkingDataSizes: true,
+  });
+  // Create a sandbox for isolated stubbing within the test
+  let sandbox = sinon.createSandbox();
+  sandbox
+    .stub(SiteDataManager, "getQuotaUsageForTimeRanges")
+    .callsFake(async () => {
+      info("stubbed getQuotaUsageForTimeRanges called");
+
+      return {
+        TIMESPAN_HOUR: 0,
+        TIMESPAN_2HOURS: 0,
+        TIMESPAN_4HOURS: LARGE_USAGE_NUM,
+        TIMESPAN_TODAY: 0,
+        TIMESPAN_EVERYTHING: 0,
+      };
+    });
+
+  dh.onload = async function () {
+    let windowObj =
+      window.browsingContext.topChromeWindow.gDialogBox._dialog._frame
+        .contentWindow;
+    let promise = new Promise(resolve => {
+      windowObj.addEventListener("resize", resolve);
+    });
+    this.selectDuration(Sanitizer.TIMESPAN_4HOURS);
+
+    await promise;
+    ok(
+      !isIframeOverflowing(windowObj.document.getElementById("SanitizeDialog")),
+      "There should be no overflow on wrapping in the dialog"
+    );
+
+    this.selectDuration(Sanitizer.TIMESPAN_2HOURS);
+    await promise;
+    ok(
+      !isIframeOverflowing(windowObj.document.getElementById("SanitizeDialog")),
+      "There should be no overflow on wrapping in the dialog"
+    );
+
+    this.cancelDialog();
+  };
+  dh.open();
+  await dh.promiseClosed;
+
+  // Restore the sandbox after the test is complete
+  sandbox.restore();
+});
diff --git a/browser/base/content/test/sanitize/head.js b/browser/base/content/test/sanitize/head.js
index 30d96c69f6..1b41226fd1 100644
--- a/browser/base/content/test/sanitize/head.js
+++ b/browser/base/content/test/sanitize/head.js
@@ -49,10 +49,10 @@ function checkIndexedDB(host, originAttributes) {
       originAttributes
     );
     let request = indexedDB.openForPrincipal(principal, "TestDatabase", 1);
-    request.onupgradeneeded = function (e) {
+    request.onupgradeneeded = function () {
       data = false;
     };
-    request.onsuccess = function (e) {
+    request.onsuccess = function () {
       resolve(data);
     };
   });
diff --git a/browser/base/content/test/sidebar/browser_sidebar_move.js b/browser/base/content/test/sidebar/browser_sidebar_move.js
index 3de26b7966..d434b3bbd8 100644
--- a/browser/base/content/test/sidebar/browser_sidebar_move.js
+++ b/browser/base/content/test/sidebar/browser_sidebar_move.js
@@ -4,15 +4,17 @@ registerCleanupFunction(() => {
 });
 
 const EXPECTED_START_ORDINALS = [
-  ["sidebar-box", 1],
-  ["sidebar-splitter", 2],
-  ["appcontent", 3],
+  ["sidebar-launcher", 1],
+  ["sidebar-box", 2],
+  ["sidebar-splitter", 3],
+  ["appcontent", 4],
 ];
 
 const EXPECTED_END_ORDINALS = [
-  ["sidebar-box", 3],
-  ["sidebar-splitter", 2],
-  ["appcontent", 1],
+  ["sidebar-launcher", 5],
+  ["sidebar-box", 4],
+  ["sidebar-splitter", 3],
+  ["appcontent", 2],
 ];
 
 function getBrowserChildrenWithOrdinals() {
diff --git a/browser/base/content/test/siteIdentity/browser_identityBlock_focus.js b/browser/base/content/test/siteIdentity/browser_identityBlock_focus.js
index 858cd3d632..79b6d216c5 100644
--- a/browser/base/content/test/siteIdentity/browser_identityBlock_focus.js
+++ b/browser/base/content/test/siteIdentity/browser_identityBlock_focus.js
@@ -98,7 +98,7 @@ add_task(async function testWithNotifications() {
 // Checks that with invalid pageproxystate the identity block is ignored.
 add_task(async function testInvalidPageProxyState() {
   await SpecialPowers.pushPrefEnv({ set: [["accessibility.tabfocus", 7]] });
-  await BrowserTestUtils.withNewTab("about:blank", async function (browser) {
+  await BrowserTestUtils.withNewTab("about:blank", async function () {
     // Loading about:blank will automatically focus the urlbar, which, however, can
     // race with the test code. So we only send the shortcut if the urlbar isn't focused yet.
     if (document.activeElement != gURLBar.inputField) {
diff --git a/browser/base/content/test/siteIdentity/browser_identityPopup_clearSiteData.js b/browser/base/content/test/siteIdentity/browser_identityPopup_clearSiteData.js
index 0107814b98..ff77b42ed8 100644
--- a/browser/base/content/test/siteIdentity/browser_identityPopup_clearSiteData.js
+++ b/browser/base/content/test/siteIdentity/browser_identityPopup_clearSiteData.js
@@ -67,7 +67,7 @@ async function testClearing(
     });
   }
 
-  await BrowserTestUtils.withNewTab(testURI, async function (browser) {
+  await BrowserTestUtils.withNewTab(testURI, async function () {
     // Verify we have added quota storage.
     if (testQuota) {
       let usage = await SiteDataTestUtils.getQuotaUsage(originA);
diff --git a/browser/base/content/test/siteIdentity/browser_navigation_failures.js b/browser/base/content/test/siteIdentity/browser_navigation_failures.js
index ac3fcc4067..f71552cdcf 100644
--- a/browser/base/content/test/siteIdentity/browser_navigation_failures.js
+++ b/browser/base/content/test/siteIdentity/browser_navigation_failures.js
@@ -85,10 +85,10 @@ function startServer(cert) {
       output = transport.openOutputStream(0, 0, 0);
     },
 
-    onHandshakeDone(socket, status) {
+    onHandshakeDone() {
       input.asyncWait(
         {
-          onInputStreamReady(readyInput) {
+          onInputStreamReady() {
             try {
               input.close();
               output.close();
diff --git a/browser/base/content/test/siteIdentity/browser_secure_transport_insecure_scheme.js b/browser/base/content/test/siteIdentity/browser_secure_transport_insecure_scheme.js
index 9dce76266a..bd004bae1b 100644
--- a/browser/base/content/test/siteIdentity/browser_secure_transport_insecure_scheme.js
+++ b/browser/base/content/test/siteIdentity/browser_secure_transport_insecure_scheme.js
@@ -19,7 +19,7 @@ const NOT_SECURE_LABEL = Services.prefs.getBoolPref(
  * @param {string} uri - URI of the page to test with.
  */
 async function testPageInfoNotEncrypted(uri) {
-  let pageInfo = BrowserPageInfo(uri, "securityTab");
+  let pageInfo = BrowserCommands.pageInfo(uri, "securityTab");
   await BrowserTestUtils.waitForEvent(pageInfo, "load");
   let pageInfoDoc = pageInfo.document;
   let securityTab = pageInfoDoc.getElementById("securityTab");
@@ -87,7 +87,7 @@ function startServer(cert) {
       output = transport.openOutputStream(0, 0, 0);
     },
 
-    onHandshakeDone(socket, status) {
+    onHandshakeDone() {
       input.asyncWait(
         {
           onInputStreamReady(readyInput) {
@@ -152,7 +152,7 @@ add_task(async function () {
     QueryInterface: ChromeUtils.generateQI(["nsISystemProxySettings"]),
     mainThreadOnly: true,
     PACURI: null,
-    getProxyForURI: (aSpec, aScheme, aHost, aPort) => {
+    getProxyForURI: () => {
       return `HTTPS localhost:${server.port}`;
     },
   };
@@ -181,7 +181,7 @@ add_task(async function () {
   // secure, in a real situation the connection from the proxy to
   // http://example.com won't be secure, so we treat it as not secure.
   // eslint-disable-next-line @microsoft/sdl/no-insecure-url
-  await BrowserTestUtils.withNewTab("http://example.com/", async browser => {
+  await BrowserTestUtils.withNewTab("http://example.com/", async () => {
     let identityMode = window.document.getElementById("identity-box").className;
     is(
       identityMode,
diff --git a/browser/base/content/test/siteIdentity/head.js b/browser/base/content/test/siteIdentity/head.js
index 733796ffb7..9936a8bf6f 100644
--- a/browser/base/content/test/siteIdentity/head.js
+++ b/browser/base/content/test/siteIdentity/head.js
@@ -244,12 +244,12 @@ async function assertMixedContentBlockingState(tabbrowser, states = {}) {
   );
   gIdentityHandler._identityIconBox.click();
   await promisePanelOpen;
-  let popupAttr = doc
-    .getElementById("identity-popup")
-    .getAttribute("mixedcontent");
-  let bodyAttr = doc
-    .getElementById("identity-popup-securityView-extended-info")
-    .getAttribute("mixedcontent");
+  let popupAttr =
+    doc.getElementById("identity-popup").getAttribute("mixedcontent") || "";
+  let bodyAttr =
+    doc
+      .getElementById("identity-popup-securityView-extended-info")
+      .getAttribute("mixedcontent") || "";
 
   is(
     popupAttr.includes("active-loaded"),
diff --git a/browser/base/content/test/static/browser_all_files_referenced.js b/browser/base/content/test/static/browser_all_files_referenced.js
index 5e83443ec7..d73c3fe8de 100644
--- a/browser/base/content/test/static/browser_all_files_referenced.js
+++ b/browser/base/content/test/static/browser_all_files_referenced.js
@@ -81,6 +81,9 @@ var gExceptionPaths = [
 
   // CSS files are referenced inside JS in an html template
   "chrome://browser/content/aboutlogins/components/",
+
+  // Strip on Share parameter lists
+  "chrome://global/content/antitracking/",
 ];
 
 // These are not part of the omni.ja file, so we find them only when running
@@ -99,13 +102,6 @@ if (AppConstants.MOZ_BACKGROUNDTASKS) {
   gExceptionPaths.push("resource://app/modules/backgroundtasks/");
 }
 
-if (AppConstants.NIGHTLY_BUILD) {
-  // This is nightly-only debug tool.
-  gExceptionPaths.push(
-    "chrome://browser/content/places/interactionsViewer.html"
-  );
-}
-
 // Each allowlist entry should have a comment indicating which file is
 // referencing the listed file in a way that the test can't detect, or a
 // bug number to remove or use the file if it is indeed currently unreferenced.
@@ -283,6 +279,9 @@ var allowlist = [
   // Bug 1875361
   { file: "chrome://global/content/ml/SummarizerModel.sys.mjs" },
 
+  // Bug 1886130
+  { file: "chrome://global/content/ml/ModelHub.sys.mjs" },
+
   // toolkit/xre/MacRunFromDmgUtils.mm
   { file: "resource://gre/localization/en-US/toolkit/global/run-from-dmg.ftl" },
 
@@ -293,6 +292,19 @@ var allowlist = [
   { file: "chrome://browser/content/screenshots/download-white.svg" },
 ];
 
+if (AppConstants.NIGHTLY_BUILD) {
+  allowlist.push(
+    ...[
+      // This is nightly-only debug tool.
+      { file: "chrome://browser/content/places/interactionsViewer.html" },
+
+      // A debug tool that is only available in Nightly builds, and is accessed
+      // directly by developers via the chrome URI (bug 1888491)
+      { file: "chrome://browser/content/backup/debug.html" },
+    ]
+  );
+}
+
 if (AppConstants.platform != "win") {
   // toolkit/mozapps/defaultagent/Notification.cpp
   // toolkit/mozapps/defaultagent/ScheduledTask.cpp
diff --git a/browser/base/content/test/static/browser_parsable_css.js b/browser/base/content/test/static/browser_parsable_css.js
index 602cc5a7e2..fb91da578a 100644
--- a/browser/base/content/test/static/browser_parsable_css.js
+++ b/browser/base/content/test/static/browser_parsable_css.js
@@ -13,12 +13,6 @@ let ignoreList = [
   // CodeMirror is imported as-is, see bug 1004423.
   { sourceName: /codemirror\.css$/i, isFromDevTools: true },
   // UA-only media features.
-  {
-    sourceName: /\b(autocomplete-item)\.css$/,
-    errorMessage: /Expected media feature name but found \u2018-moz.*/i,
-    isFromDevTools: false,
-    platforms: ["windows"],
-  },
   {
     sourceName:
       /\b(contenteditable|EditorOverride|svg|forms|html|mathml|ua)\.css$/i,
@@ -27,7 +21,7 @@ let ignoreList = [
   },
   {
     sourceName:
-      /\b(scrollbars|xul|html|mathml|ua|forms|svg|manageDialog|autocomplete-item-shared|formautofill)\.css$/i,
+      /\b(scrollbars|xul|html|mathml|ua|forms|svg|manageDialog|formautofill)\.css$/i,
     errorMessage: /Unknown property.*-moz-/i,
     isFromDevTools: false,
   },
@@ -123,6 +117,8 @@ let propNameAllowlist = [
     isFromDevTools: false,
   },
   { propName: "--browser-stack-z-index-rdm-toolbar", isFromDevTools: false },
+  // about:profiling is in devtools even though it uses non-devtools styles.
+  { propName: "--in-content-border-hover", isFromDevTools: false },
 
   // These variables are specified from devtools but read from non-devtools
   // styles, which confuses the test.
@@ -434,13 +430,13 @@ add_task(async function checkAllTheCSS() {
   let loadCSS = chromeUri =>
     new Promise(resolve => {
       let linkEl, onLoad, onError;
-      onLoad = e => {
+      onLoad = () => {
         processCSSRules(linkEl.sheet);
         resolve();
         linkEl.removeEventListener("load", onLoad);
         linkEl.removeEventListener("error", onError);
       };
-      onError = e => {
+      onError = () => {
         ok(
           false,
           "Loading " + linkEl.getAttribute("href") + " threw an error!"
diff --git a/browser/base/content/test/static/browser_parsable_script.js b/browser/base/content/test/static/browser_parsable_script.js
index d4dcbd87fe..3d8fbc1535 100644
--- a/browser/base/content/test/static/browser_parsable_script.js
+++ b/browser/base/content/test/static/browser_parsable_script.js
@@ -22,9 +22,9 @@ const kESModuleList = new Set([
   /chrome\/pdfjs\/content\/web\/.*\.js$/,
 ]);
 
-// Normally we would use reflect.jsm to get Reflect.parse. However, if
-// we do that, then all the AST data is allocated in reflect.jsm's
-// zone. That exposes a bug in our GC. The GC collects reflect.jsm's
+// Normally we would use reflect.sys.mjs to get Reflect.parse. However, if
+// we do that, then all the AST data is allocated in reflect.sys.mjs's
+// zone. That exposes a bug in our GC. The GC collects reflect.sys.mjs's
 // zone but not the zone in which our test code lives (since no new
 // data is being allocated in it). The cross-compartment wrappers in
 // our zone that point to the AST data never get collected, and so the
@@ -69,7 +69,7 @@ function uriIsESModule(uri) {
 }
 
 function parsePromise(uri, parseTarget) {
-  let promise = new Promise((resolve, reject) => {
+  let promise = new Promise(resolve => {
     let xhr = new XMLHttpRequest();
     xhr.open("GET", uri, true);
     xhr.onreadystatechange = function () {
diff --git a/browser/base/content/test/static/head.js b/browser/base/content/test/static/head.js
index d9b978e853..317ad430af 100644
--- a/browser/base/content/test/static/head.js
+++ b/browser/base/content/test/static/head.js
@@ -135,7 +135,7 @@ function* generateEntriesFromJarFile(jarFile, extension) {
 }
 
 function fetchFile(uri) {
-  return new Promise((resolve, reject) => {
+  return new Promise(resolve => {
     let xhr = new XMLHttpRequest();
     xhr.responseType = "text";
     xhr.open("GET", uri, true);
diff --git a/browser/base/content/test/sync/browser_contextmenu_sendtab.js b/browser/base/content/test/sync/browser_contextmenu_sendtab.js
index 4922869c1d..bc49680e37 100644
--- a/browser/base/content/test/sync/browser_contextmenu_sendtab.js
+++ b/browser/base/content/test/sync/browser_contextmenu_sendtab.js
@@ -323,7 +323,7 @@ async function openTabContextMenu(openSubmenuId = null) {
 
 function promiseObserver(topic) {
   return new Promise(resolve => {
-    let obs = (aSubject, aTopic, aData) => {
+    let obs = (aSubject, aTopic) => {
       Services.obs.removeObserver(obs, aTopic);
       resolve(aSubject);
     };
diff --git a/browser/base/content/test/sync/browser_fxa_web_channel.js b/browser/base/content/test/sync/browser_fxa_web_channel.js
index c232f26f26..23c5510422 100644
--- a/browser/base/content/test/sync/browser_fxa_web_channel.js
+++ b/browser/base/content/test/sync/browser_fxa_web_channel.js
@@ -27,7 +27,7 @@ var gTests = [
         content_uri: TEST_HTTP_PATH,
         channel_id: TEST_CHANNEL_ID,
       });
-      let promiseObserver = new Promise((resolve, reject) => {
+      let promiseObserver = new Promise(resolve => {
         makeObserver(
           ON_PROFILE_CHANGE_NOTIFICATION,
           function (subject, topic, data) {
@@ -52,7 +52,7 @@ var gTests = [
   {
     desc: "fxa web channel - login messages should notify the fxAccounts object",
     async run() {
-      let promiseLogin = new Promise((resolve, reject) => {
+      let promiseLogin = new Promise(resolve => {
         let login = accountData => {
           Assert.equal(typeof accountData.authAt, "number");
           Assert.equal(accountData.email, "testuser@testuser.com");
@@ -91,7 +91,7 @@ var gTests = [
     async run() {
       let properUrl = TEST_BASE_URL + "?can_link_account";
 
-      let promiseEcho = new Promise((resolve, reject) => {
+      let promiseEcho = new Promise(resolve => {
         let webChannelOrigin = Services.io.newURI(properUrl);
         // responses sent to content are echoed back over the
         // `fxaccounts_webchannel_response_echo` channel. Ensure the
@@ -100,7 +100,7 @@ var gTests = [
           "fxaccounts_webchannel_response_echo",
           webChannelOrigin
         );
-        echoWebChannel.listen((webChannelId, message, target) => {
+        echoWebChannel.listen((webChannelId, message) => {
           Assert.equal(message.command, "fxaccounts:can_link_account");
           Assert.equal(message.messageId, 2);
           Assert.equal(message.data.ok, true);
@@ -136,7 +136,7 @@ var gTests = [
   {
     desc: "fxa web channel - logout messages should notify the fxAccounts object",
     async run() {
-      let promiseLogout = new Promise((resolve, reject) => {
+      let promiseLogout = new Promise(resolve => {
         let logout = uid => {
           Assert.equal(uid, "uid");
 
@@ -167,7 +167,7 @@ var gTests = [
   {
     desc: "fxa web channel - delete messages should notify the fxAccounts object",
     async run() {
-      let promiseDelete = new Promise((resolve, reject) => {
+      let promiseDelete = new Promise(resolve => {
         let logout = uid => {
           Assert.equal(uid, "uid");
 
@@ -199,8 +199,8 @@ var gTests = [
     desc: "fxa web channel - firefox_view messages should call the openFirefoxView helper",
     async run() {
       let wasCalled = false;
-      let promiseMessageHandled = new Promise((resolve, reject) => {
-        let openFirefoxView = (browser, entryPoint) => {
+      let promiseMessageHandled = new Promise(resolve => {
+        let openFirefoxView = browser => {
           wasCalled = true;
           Assert.ok(
             !!browser.ownerGlobal,
diff --git a/browser/base/content/test/sync/browser_sync.js b/browser/base/content/test/sync/browser_sync.js
index 168c6f22b0..8f687842a4 100644
--- a/browser/base/content/test/sync/browser_sync.js
+++ b/browser/base/content/test/sync/browser_sync.js
@@ -772,7 +772,7 @@ function checkSyncNowButtons(syncing, tooltip = null) {
   for (const syncButton of syncButtons) {
     is(
       syncButton.getAttribute("syncstatus"),
-      syncing ? "active" : "",
+      syncing ? "active" : null,
       "button active has the right value"
     );
     if (tooltip) {
@@ -894,7 +894,7 @@ function checkItemsVisibilities(itemsIds, expectedShownItemId) {
 
 function promiseObserver(topic) {
   return new Promise(resolve => {
-    let obs = (aSubject, aTopic, aData) => {
+    let obs = (aSubject, aTopic) => {
       Services.obs.removeObserver(obs, aTopic);
       resolve(aSubject);
     };
diff --git a/browser/base/content/test/tabPrompts/browser_abort_when_in_modal_state.js b/browser/base/content/test/tabPrompts/browser_abort_when_in_modal_state.js
index cb3a1f72d6..d6d2434017 100644
--- a/browser/base/content/test/tabPrompts/browser_abort_when_in_modal_state.js
+++ b/browser/base/content/test/tabPrompts/browser_abort_when_in_modal_state.js
@@ -8,14 +8,10 @@ const { PromiseTestUtils } = ChromeUtils.importESModule(
 );
 
 /**
- * Check that if we're using a window-modal prompt,
- * the next synchronous window-internal modal prompt aborts rather than
- * leaving us in a deadlock about how to enter modal state.
+ * Check that the next synchronous window-internal modal prompt aborts rather
+ * than leaving us in a deadlock about how to enter modal state.
  */
 add_task(async function test_check_multiple_prompts() {
-  await SpecialPowers.pushPrefEnv({
-    set: [["prompts.windowPromptSubDialog", true]],
-  });
   let container = document.getElementById("window-modal-dialog");
   let dialogPromise = BrowserTestUtils.promiseAlertDialogOpen();
 
diff --git a/browser/base/content/test/tabPrompts/browser_auth_spoofing_protection.js b/browser/base/content/test/tabPrompts/browser_auth_spoofing_protection.js
index 86d7c992c5..1211694973 100644
--- a/browser/base/content/test/tabPrompts/browser_auth_spoofing_protection.js
+++ b/browser/base/content/test/tabPrompts/browser_auth_spoofing_protection.js
@@ -119,7 +119,7 @@ async function waitForDialog(doConfirmPrompt, crossDomain, prefEnabled) {
     } else {
       Assert.equal(
         dialog._overlay.getAttribute("hideContent"),
-        "",
+        null,
         "Dialog overlay does not hide the current sites content"
       );
       Assert.equal(
@@ -137,7 +137,7 @@ async function waitForDialog(doConfirmPrompt, crossDomain, prefEnabled) {
   } else {
     Assert.equal(
       dialog._overlay.getAttribute("hideContent"),
-      "",
+      null,
       "Dialog overlay does not hide the current sites content"
     );
     Assert.equal(
diff --git a/browser/base/content/test/tabPrompts/browser_contentOrigins.js b/browser/base/content/test/tabPrompts/browser_contentOrigins.js
index 2bf4ba6039..0c40763a99 100644
--- a/browser/base/content/test/tabPrompts/browser_contentOrigins.js
+++ b/browser/base/content/test/tabPrompts/browser_contentOrigins.js
@@ -127,12 +127,6 @@ async function checkDialog(
   });
 }
 
-add_setup(async function () {
-  await SpecialPowers.pushPrefEnv({
-    set: [["prompts.modalType.httpAuth", Ci.nsIPrompt.MODAL_TYPE_TAB]],
-  });
-});
-
 add_task(async function test_check_prompt_origin_display() {
   await checkAlert("https://example.com/", { value: "example.com" });
   // eslint-disable-next-line @microsoft/sdl/no-insecure-url
diff --git a/browser/base/content/test/tabPrompts/browser_windowPrompt.js b/browser/base/content/test/tabPrompts/browser_windowPrompt.js
index 535142f485..0aca489b50 100644
--- a/browser/base/content/test/tabPrompts/browser_windowPrompt.js
+++ b/browser/base/content/test/tabPrompts/browser_windowPrompt.js
@@ -7,9 +7,6 @@
  * Check that the in-window modal dialogs work correctly.
  */
 add_task(async function test_check_window_modal_prompt_service() {
-  await SpecialPowers.pushPrefEnv({
-    set: [["prompts.windowPromptSubDialog", true]],
-  });
   let dialogPromise = BrowserTestUtils.promiseAlertDialogOpen();
   // Avoid blocking the test on the (sync) alert by sticking it in a timeout:
   setTimeout(
@@ -69,9 +66,6 @@ add_task(async function test_check_window_modal_prompt_service() {
  * Check that the dialog's own closing methods being invoked don't break things.
  */
 add_task(async function test_check_window_modal_prompt_service() {
-  await SpecialPowers.pushPrefEnv({
-    set: [["prompts.windowPromptSubDialog", true]],
-  });
   let dialogPromise = BrowserTestUtils.promiseAlertDialogOpen();
   // Avoid blocking the test on the (sync) alert by sticking it in a timeout:
   setTimeout(
@@ -105,9 +99,6 @@ add_task(async function test_check_window_modal_prompt_service() {
 });
 
 add_task(async function test_check_multiple_prompts() {
-  await SpecialPowers.pushPrefEnv({
-    set: [["prompts.windowPromptSubDialog", true]],
-  });
   let container = document.getElementById("window-modal-dialog");
   let dialogPromise = BrowserTestUtils.promiseAlertDialogOpen();
 
@@ -173,9 +164,6 @@ add_task(async function test_check_minimize_response() {
   if (AppConstants.platform == "linux") {
     return;
   }
-  await SpecialPowers.pushPrefEnv({
-    set: [["prompts.windowPromptSubDialog", true]],
-  });
 
   let promiseSizeModeChange = BrowserTestUtils.waitForEvent(
     window,
@@ -235,10 +223,6 @@ add_task(async function test_check_minimize_response() {
  * underlying SubDialog has fully opened.
  */
 add_task(async function test_closed_callback() {
-  await SpecialPowers.pushPrefEnv({
-    set: [["prompts.windowPromptSubDialog", true]],
-  });
-
   let promptClosedPromise = Services.prompt.asyncAlert(
     window.browsingContext,
     Services.prompt.MODAL_TYPE_INTERNAL_WINDOW,
diff --git a/browser/base/content/test/tabcrashed/browser_aboutRestartRequired.toml b/browser/base/content/test/tabcrashed/browser_aboutRestartRequired.toml
index 88988434fb..8f8648d810 100644
--- a/browser/base/content/test/tabcrashed/browser_aboutRestartRequired.toml
+++ b/browser/base/content/test/tabcrashed/browser_aboutRestartRequired.toml
@@ -14,8 +14,10 @@ prefs = [
 #["browser_aboutRestartRequired_buildid_false-positive.js"]
 #skip-if = ["win11_2009 && msix && debug"]  # bug 1823581
 
-["browser_aboutRestartRequired_buildid_mismatch.js"]
-skip-if = ["win11_2009 && msix && debug"]  # bug 1823581
+# Bug 1888355: re-enable once bug 1877361 is fixed
+#["browser_aboutRestartRequired_buildid_mismatch.js"]
+#skip-if = ["win11_2009 && msix && debug"]  # bug 1823581
 
-["browser_aboutRestartRequired_buildid_no-platform-ini.js"]
-skip-if = ["win11_2009 && msix && debug"]  # bug 1823581
+# Bug 1888355: re-enable once bug 1877361 is fixed
+#["browser_aboutRestartRequired_buildid_no-platform-ini.js"]
+#skip-if = ["win11_2009 && msix && debug"]  # bug 1823581
diff --git a/browser/base/content/test/tabcrashed/browser_aboutRestartRequired_noForkServer.toml b/browser/base/content/test/tabcrashed/browser_aboutRestartRequired_noForkServer.toml
index ec045ddf79..ddb49a0e26 100644
--- a/browser/base/content/test/tabcrashed/browser_aboutRestartRequired_noForkServer.toml
+++ b/browser/base/content/test/tabcrashed/browser_aboutRestartRequired_noForkServer.toml
@@ -12,3 +12,11 @@ prefs = [
 # Bug 1876056: remove once bug 1877361 is fixed
 ["browser_aboutRestartRequired_buildid_false-positive.js"]
 skip-if = ["win11_2009 && msix && debug"]  # bug 1823581
+
+# Bug 1888355: re-enable once bug 1877361 is fixed
+["browser_aboutRestartRequired_buildid_mismatch.js"]
+skip-if = ["win11_2009 && msix && debug"]  # bug 1823581
+
+# Bug 1888355: re-enable once bug 1877361 is fixed
+["browser_aboutRestartRequired_buildid_no-platform-ini.js"]
+skip-if = ["win11_2009 && msix && debug"]  # bug 1823581
diff --git a/browser/base/content/test/tabcrashed/head.js b/browser/base/content/test/tabcrashed/head.js
index bb57c85d6d..b4e9137012 100644
--- a/browser/base/content/test/tabcrashed/head.js
+++ b/browser/base/content/test/tabcrashed/head.js
@@ -117,7 +117,7 @@ async function setupLocalCrashReportServer() {
   // reports.  This test needs them enabled.  The test also needs a mock
   // report server, and fortunately one is already set up by toolkit/
   // crashreporter/test/Makefile.in.  Assign its URL to MOZ_CRASHREPORTER_URL,
-  // which CrashSubmit.jsm uses as a server override.
+  // which CrashSubmit.sys.mjs uses as a server override.
   let noReport = Services.env.get("MOZ_CRASHREPORTER_NO_REPORT");
   let serverUrl = Services.env.get("MOZ_CRASHREPORTER_URL");
   Services.env.set("MOZ_CRASHREPORTER_NO_REPORT", "");
@@ -135,7 +135,7 @@ async function setupLocalCrashReportServer() {
  */
 function prepareNoDump() {
   let originalGetDumpID = TabCrashHandler.getDumpID;
-  TabCrashHandler.getDumpID = function (browser) {
+  TabCrashHandler.getDumpID = function () {
     return null;
   };
   registerCleanupFunction(() => {
@@ -156,11 +156,11 @@ function unsetBuildidMatchDontSendEnv() {
 }
 
 function getEventPromise(eventName, eventKind) {
-  return new Promise(function (resolve, reject) {
+  return new Promise(function (resolve) {
     info("Installing event listener (" + eventKind + ")");
     window.addEventListener(
       eventName,
-      event => {
+      () => {
         ok(true, "Received " + eventName + " (" + eventKind + ") event");
         info("Call resolve() for " + eventKind + " event");
         resolve();
diff --git a/browser/base/content/test/tabs/browser.toml b/browser/base/content/test/tabs/browser.toml
index 1b4a6900bf..fa77a8b1a4 100644
--- a/browser/base/content/test/tabs/browser.toml
+++ b/browser/base/content/test/tabs/browser.toml
@@ -76,6 +76,8 @@ support-files = ["tab_that_closes.html"]
 
 ["browser_hiddentab_contextmenu.js"]
 
+["browser_lastSeenActive.js"]
+
 ["browser_lazy_tab_browser_events.js"]
 
 ["browser_link_in_tab_title_and_url_prefilled_blank_page.js"]
diff --git a/browser/base/content/test/tabs/browser_allow_process_switches_despite_related_browser.js b/browser/base/content/test/tabs/browser_allow_process_switches_despite_related_browser.js
index b782c3aada..fea1de8fe0 100644
--- a/browser/base/content/test/tabs/browser_allow_process_switches_despite_related_browser.js
+++ b/browser/base/content/test/tabs/browser_allow_process_switches_despite_related_browser.js
@@ -13,7 +13,7 @@ add_task(async function () {
   });
 
   let promiseTab = BrowserTestUtils.waitForNewTab(gBrowser, DATA_URI_SOURCE);
-  BrowserViewSource(tab.linkedBrowser);
+  BrowserCommands.viewSource(tab.linkedBrowser);
   let viewSourceTab = await promiseTab;
   registerCleanupFunction(async function () {
     BrowserTestUtils.removeTab(viewSourceTab);
diff --git a/browser/base/content/test/tabs/browser_audioTabIcon.js b/browser/base/content/test/tabs/browser_audioTabIcon.js
index 53b5140abb..c065e2b173 100644
--- a/browser/base/content/test/tabs/browser_audioTabIcon.js
+++ b/browser/base/content/test/tabs/browser_audioTabIcon.js
@@ -396,7 +396,7 @@ async function test_swapped_browser_while_not_playing(oldTab, newBrowser) {
   );
 
   let AudioPlaybackPromise = new Promise(resolve => {
-    let observer = (subject, topic, data) => {
+    let observer = () => {
       ok(false, "Should not see an audio-playback notification");
     };
     Services.obs.addObserver(observer, "audio-playback");
@@ -443,7 +443,7 @@ async function test_swapped_browser_while_not_playing(oldTab, newBrowser) {
   await test_tooltip(newTab.overlayIcon, "Unmute tab", true, newTab);
 }
 
-async function test_browser_swapping(tab, browser) {
+async function test_browser_swapping(tab) {
   // First, test swapping with a playing but muted tab.
   await play(tab);
 
diff --git a/browser/base/content/test/tabs/browser_e10s_about_page_triggeringprincipal.js b/browser/base/content/test/tabs/browser_e10s_about_page_triggeringprincipal.js
index 4610551977..8fbee64db4 100644
--- a/browser/base/content/test/tabs/browser_e10s_about_page_triggeringprincipal.js
+++ b/browser/base/content/test/tabs/browser_e10s_about_page_triggeringprincipal.js
@@ -86,7 +86,7 @@ add_task(async function test_principal_ctrl_click() {
 
   await BrowserTestUtils.withNewTab(
     "about:test-about-principal-parent",
-    async function (browser) {
+    async function () {
       let loadPromise = BrowserTestUtils.waitForNewTab(
         gBrowser,
         "about:test-about-principal-child",
@@ -149,7 +149,7 @@ add_task(async function test_principal_right_click_open_link_in_new_tab() {
 
   await BrowserTestUtils.withNewTab(
     "about:test-about-principal-parent",
-    async function (browser) {
+    async function () {
       let loadPromise = BrowserTestUtils.waitForNewTab(
         gBrowser,
         "about:test-about-principal-child",
diff --git a/browser/base/content/test/tabs/browser_e10s_about_process.js b/browser/base/content/test/tabs/browser_e10s_about_process.js
index f73e8e659c..504dfe0265 100644
--- a/browser/base/content/test/tabs/browser_e10s_about_process.js
+++ b/browser/base/content/test/tabs/browser_e10s_about_process.js
@@ -37,7 +37,7 @@ const TEST_MODULES = [
 function AboutModule() {}
 
 AboutModule.prototype = {
-  newChannel(aURI, aLoadInfo) {
+  newChannel() {
     throw Components.Exception("", Cr.NS_ERROR_NOT_IMPLEMENTED);
   },
 
@@ -52,7 +52,7 @@ AboutModule.prototype = {
     return 0;
   },
 
-  getIndexedDBOriginPostfix(aURI) {
+  getIndexedDBOriginPostfix() {
     return null;
   },
 
diff --git a/browser/base/content/test/tabs/browser_lastSeenActive.js b/browser/base/content/test/tabs/browser_lastSeenActive.js
new file mode 100644
index 0000000000..d6bba57d93
--- /dev/null
+++ b/browser/base/content/test/tabs/browser_lastSeenActive.js
@@ -0,0 +1,260 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { SessionStoreTestUtils } = ChromeUtils.importESModule(
+  "resource://testing-common/SessionStoreTestUtils.sys.mjs"
+);
+
+const triggeringPrincipal_base64 = E10SUtils.SERIALIZED_SYSTEMPRINCIPAL;
+
+SessionStoreTestUtils.init(this, window);
+// take a state snapshot we can restore to after each test
+const ORIG_STATE = SessionStore.getBrowserState();
+
+const SECOND_MS = 1000;
+const DAY_MS = 24 * 60 * 60 * 1000;
+const today = new Date().getTime();
+const yesterday = new Date(Date.now() - DAY_MS).getTime();
+
+function tabEntry(url, lastAccessed) {
+  return {
+    entries: [{ url, triggeringPrincipal_base64 }],
+    lastAccessed,
+  };
+}
+
+/**
+ * Make the given window focused and active
+ */
+async function switchToWindow(win) {
+  info("switchToWindow, waiting for promiseFocus");
+  await SimpleTest.promiseFocus(win);
+  info("switchToWindow, waiting for correct Services.focus.activeWindow");
+  await BrowserTestUtils.waitForCondition(
+    () => Services.focus.activeWindow == win
+  );
+}
+
+async function changeSizeMode(win, mode) {
+  let promise = BrowserTestUtils.waitForEvent(win, "sizemodechange");
+  win[mode]();
+  await promise;
+}
+
+async function cleanup() {
+  await switchToWindow(window);
+  await SessionStoreTestUtils.promiseBrowserState(ORIG_STATE);
+  is(
+    BrowserWindowTracker.orderedWindows.length,
+    1,
+    "One window at the end of test cleanup"
+  );
+  info("cleanup, browser state restored");
+}
+
+function deltaTime(time, expectedTime) {
+  return Math.abs(expectedTime - time);
+}
+
+function getWindowUrl(win) {
+  return win.gBrowser.selectedBrowser?.currentURI?.spec;
+}
+
+function getWindowByTabUrl(url) {
+  return BrowserWindowTracker.orderedWindows.find(
+    win => getWindowUrl(win) == url
+  );
+}
+
+add_task(async function restoredTabs() {
+  const now = Date.now();
+  await SessionStoreTestUtils.promiseBrowserState({
+    windows: [
+      {
+        tabs: [
+          tabEntry("data:,Window0-Tab0", yesterday),
+          tabEntry("data:,Window0-Tab1", yesterday),
+        ],
+        selected: 2,
+      },
+    ],
+  });
+  is(
+    gBrowser.visibleTabs[1],
+    gBrowser.selectedTab,
+    "The selected tab is the 2nd visible tab"
+  );
+  is(
+    getWindowUrl(window),
+    "data:,Window0-Tab1",
+    "The expected tab is selected"
+  );
+  Assert.greaterOrEqual(
+    gBrowser.selectedTab.lastSeenActive,
+    now,
+    "The selected tab's lastSeenActive is now"
+  );
+  Assert.greaterOrEqual(
+    gBrowser.selectedTab.lastAccessed,
+    now,
+    "The selected tab's lastAccessed is now"
+  );
+
+  // tab restored from session but never seen or active
+  is(
+    gBrowser.visibleTabs[0].lastSeenActive,
+    yesterday,
+    "The restored tab's lastSeenActive is yesterday"
+  );
+  await cleanup();
+});
+
+add_task(async function switchingTabs() {
+  let now = Date.now();
+  let initialTab = gBrowser.selectedTab;
+  let applicationStart = Services.startup.getStartupInfo().start.getTime();
+  let openedTab = BrowserTestUtils.addTab(gBrowser, "data:,Tab1");
+  await BrowserTestUtils.browserLoaded(openedTab.linkedBrowser);
+
+  ok(!openedTab.selected, "The background tab we opened isn't selected");
+  Assert.greaterOrEqual(
+    initialTab.selected && initialTab.lastSeenActive,
+    now,
+    "The initial tab is selected and last seen now"
+  );
+
+  is(
+    openedTab.lastSeenActive,
+    applicationStart,
+    `Background tab got default lastSeenActive value, delta: ${deltaTime(
+      openedTab.lastSeenActive,
+      applicationStart
+    )}`
+  );
+
+  now = Date.now();
+  await BrowserTestUtils.switchTab(gBrowser, openedTab);
+  Assert.greaterOrEqual(
+    openedTab.lastSeenActive,
+    now,
+    "The tab we switched to is last seen now"
+  );
+
+  await cleanup();
+});
+
+add_task(async function switchingWindows() {
+  info("Restoring to the test browser state");
+  await SessionStoreTestUtils.promiseBrowserState({
+    windows: [
+      {
+        tabs: [tabEntry("data:,Window1-Tab0", yesterday)],
+        selected: 1,
+        sizemodeBeforeMinimized: "normal",
+        sizemode: "maximized",
+        zIndex: 1, // this will be the selected window
+      },
+      {
+        tabs: [tabEntry("data:,Window2-Tab0", yesterday)],
+        selected: 1,
+        sizemodeBeforeMinimized: "normal",
+        sizemode: "maximized",
+        zIndex: 2,
+      },
+    ],
+  });
+  info("promiseBrowserState resolved");
+  info(
+    `BrowserWindowTracker.pendingWindows: ${BrowserWindowTracker.pendingWindows.size}`
+  );
+  await Promise.all(
+    Array.from(BrowserWindowTracker.pendingWindows.values()).map(
+      win => win.deferred.promise
+    )
+  );
+  info("All the pending windows are resolved");
+  info("Waiting for the firstBrowserLoaded in each of the windows");
+  await Promise.all(
+    BrowserWindowTracker.orderedWindows.map(win => {
+      const selectedUrl = getWindowUrl(win);
+      if (selectedUrl && selectedUrl !== "about:blank") {
+        return Promise.resolve();
+      }
+      return BrowserTestUtils.firstBrowserLoaded(win, false);
+    })
+  );
+  let expectedTabURLs = ["data:,Window1-Tab0", "data:,Window2-Tab0"];
+  let [win1, win2] = expectedTabURLs.map(url => getWindowByTabUrl(url));
+  if (BrowserWindowTracker.getTopWindow() !== win1) {
+    info("Switch to win1 which isn't active/top after restoring session");
+    // In theory the zIndex values in the session state should make win1 active
+    // But in practice that isn't always true. To ensure we're testing from a known state,
+    // ensure the first window is active before proceeding with the test
+    await switchToWindow(win1);
+    [win1, win2] = expectedTabURLs.map(url => getWindowByTabUrl(url));
+  }
+
+  let actualTabURLs = Array.from(BrowserWindowTracker.orderedWindows).map(win =>
+    getWindowUrl(win)
+  );
+  Assert.deepEqual(
+    actualTabURLs,
+    expectedTabURLs,
+    "Both windows are open with selected tab URLs in the expected order"
+  );
+
+  let lastSeenTimes = [win1, win2].map(
+    win => win.gBrowser.selectedTab.lastSeenActive
+  );
+
+  info("Focusing the other window");
+  await switchToWindow(win2);
+  // wait a little so the timestamps will differ and then check again
+  // eslint-disable-next-line mozilla/no-arbitrary-setTimeout
+  await new Promise(res => setTimeout(res, 100));
+  Assert.greater(
+    win2.gBrowser.selectedTab.lastSeenActive,
+    lastSeenTimes[1],
+    "The foreground window selected tab is last seen more recently than it was before being focused"
+  );
+  Assert.greater(
+    win2.gBrowser.selectedTab.lastSeenActive,
+    win1.gBrowser.selectedTab.lastSeenActive,
+    "The foreground window selected tab is last seen more recently than the backgrounded one"
+  );
+
+  lastSeenTimes = [win1, win2].map(
+    win => win.gBrowser.selectedTab.lastSeenActive
+  );
+  // minimize the foreground window and focus the other
+  let promiseSizeModeChange = BrowserTestUtils.waitForEvent(
+    win2,
+    "sizemodechange"
+  );
+  win2.minimize();
+  info("Waiting for the sizemodechange on minimized window");
+  await promiseSizeModeChange;
+  await switchToWindow(win1);
+
+  ok(
+    !win2.gBrowser.selectedTab.linkedBrowser.docShellIsActive,
+    "Docshell should be Inactive"
+  );
+  ok(win2.document.hidden, "Minimized windows's document should be hidden");
+
+  // wait a little so the timestamps will differ and then check again
+  // eslint-disable-next-line mozilla/no-arbitrary-setTimeout
+  await new Promise(res => setTimeout(res, 100));
+  Assert.greater(
+    win1.gBrowser.selectedTab.lastSeenActive,
+    win2.gBrowser.selectedTab.lastSeenActive,
+    "The foreground window selected tab is last seen more recently than the minimized one"
+  );
+  Assert.greater(
+    win1.gBrowser.selectedTab.lastSeenActive,
+    lastSeenTimes[0],
+    "The foreground window selected tab is last seen more recently than it was before being focused"
+  );
+
+  await cleanup();
+});
diff --git a/browser/base/content/test/tabs/browser_lazy_tab_browser_events.js b/browser/base/content/test/tabs/browser_lazy_tab_browser_events.js
index 665bdb7f69..5e09225cde 100644
--- a/browser/base/content/test/tabs/browser_lazy_tab_browser_events.js
+++ b/browser/base/content/test/tabs/browser_lazy_tab_browser_events.js
@@ -93,10 +93,10 @@ add_task(async function test_hidden_muted_lazy_tabs_and_swapping() {
   mutedTab.toggleMuteAudio();
   gBrowser.hideTab(hiddenTab);
 
-  is(lazyTab.linkedPanel, "", "lazyTab is lazy");
-  is(hiddenTab.linkedPanel, "", "hiddenTab is lazy");
-  is(mutedTab.linkedPanel, "", "mutedTab is lazy");
-  is(normalTab.linkedPanel, "", "normalTab is lazy");
+  is(lazyTab.linkedPanel, null, "lazyTab is lazy");
+  is(hiddenTab.linkedPanel, null, "hiddenTab is lazy");
+  is(mutedTab.linkedPanel, null, "mutedTab is lazy");
+  is(normalTab.linkedPanel, null, "normalTab is lazy");
 
   ok(mutedTab.linkedBrowser.audioMuted, "mutedTab is muted");
   ok(hiddenTab.hidden, "hiddenTab is hidden");
@@ -117,7 +117,7 @@ add_task(async function test_hidden_muted_lazy_tabs_and_swapping() {
   });
   gBrowser.swapBrowsersAndCloseOther(lazyTab, mutedTab);
   tabEventTracker.checkExpectations();
-  is(lazyTab.linkedPanel, "", "muted lazyTab is still lazy");
+  is(lazyTab.linkedPanel, null, "muted lazyTab is still lazy");
   ok(lazyTab.linkedBrowser.audioMuted, "muted lazyTab is now muted");
   ok(!lazyTab.hidden, "muted lazyTab is not hidden");
 
@@ -133,7 +133,7 @@ add_task(async function test_hidden_muted_lazy_tabs_and_swapping() {
   });
   gBrowser.swapBrowsersAndCloseOther(lazyTab, hiddenTab);
   tabEventTracker.checkExpectations();
-  is(lazyTab.linkedPanel, "", "hidden lazyTab is still lazy");
+  is(lazyTab.linkedPanel, null, "hidden lazyTab is still lazy");
   ok(!lazyTab.linkedBrowser.audioMuted, "hidden lazyTab is not muted any more");
   ok(lazyTab.hidden, "hidden lazyTab has been hidden");
 
@@ -149,7 +149,7 @@ add_task(async function test_hidden_muted_lazy_tabs_and_swapping() {
   });
   gBrowser.swapBrowsersAndCloseOther(lazyTab, normalTab);
   tabEventTracker.checkExpectations();
-  is(lazyTab.linkedPanel, "", "normal lazyTab is still lazy");
+  is(lazyTab.linkedPanel, null, "normal lazyTab is still lazy");
   ok(!lazyTab.linkedBrowser.audioMuted, "normal lazyTab is not muted any more");
   ok(!lazyTab.hidden, "normal lazyTab is not hidden any more");
 
diff --git a/browser/base/content/test/tabs/browser_link_in_tab_title_and_url_prefilled_normal_page_blank_target.js b/browser/base/content/test/tabs/browser_link_in_tab_title_and_url_prefilled_normal_page_blank_target.js
index f8773e3720..9212667a35 100644
--- a/browser/base/content/test/tabs/browser_link_in_tab_title_and_url_prefilled_normal_page_blank_target.js
+++ b/browser/base/content/test/tabs/browser_link_in_tab_title_and_url_prefilled_normal_page_blank_target.js
@@ -66,7 +66,7 @@ add_task(async function normal_page__foreground__abort() {
       tab: WAIT_A_BIT_LOADING_TITLE,
       urlbar: UrlbarTestUtils.trimURL(WAIT_A_BIT_URL),
     },
-    async actionWhileLoading(onTabLoaded) {
+    async actionWhileLoading() {
       info("Abort loading");
       document.getElementById("stop-button").click();
     },
@@ -160,7 +160,7 @@ add_task(async function normal_page__background__abort() {
       tab: WAIT_A_BIT_LOADING_TITLE,
       urlbar: UrlbarTestUtils.trimURL(HOME_URL),
     },
-    async actionWhileLoading(onTabLoaded) {
+    async actionWhileLoading() {
       info("Abort loading");
       document.getElementById("stop-button").click();
     },
diff --git a/browser/base/content/test/tabs/browser_link_in_tab_title_and_url_prefilled_normal_page_by_script.js b/browser/base/content/test/tabs/browser_link_in_tab_title_and_url_prefilled_normal_page_by_script.js
index 07cf7a8ea2..57e28ca834 100644
--- a/browser/base/content/test/tabs/browser_link_in_tab_title_and_url_prefilled_normal_page_by_script.js
+++ b/browser/base/content/test/tabs/browser_link_in_tab_title_and_url_prefilled_normal_page_by_script.js
@@ -44,7 +44,7 @@ add_task(async function normal_page__by_script__abort() {
       tab: BLANK_TITLE,
       urlbar: UrlbarTestUtils.trimURL(BLANK_URL),
     },
-    async actionWhileLoading(onTabLoaded) {
+    async actionWhileLoading() {
       info("Abort loading");
       document.getElementById("stop-button").click();
     },
diff --git a/browser/base/content/test/tabs/browser_link_in_tab_title_and_url_prefilled_normal_page_no_target.js b/browser/base/content/test/tabs/browser_link_in_tab_title_and_url_prefilled_normal_page_no_target.js
index ab18d7c7e0..464a7c43de 100644
--- a/browser/base/content/test/tabs/browser_link_in_tab_title_and_url_prefilled_normal_page_no_target.js
+++ b/browser/base/content/test/tabs/browser_link_in_tab_title_and_url_prefilled_normal_page_no_target.js
@@ -47,7 +47,7 @@ add_task(async function normal_page__no_target__abort() {
       tab: HOME_TITLE,
       urlbar: UrlbarTestUtils.trimURL(HOME_URL),
     },
-    async actionWhileLoading(onTabLoaded) {
+    async actionWhileLoading() {
       info("Abort loading");
       document.getElementById("stop-button").click();
     },
diff --git a/browser/base/content/test/tabs/browser_link_in_tab_title_and_url_prefilled_normal_page_other_target.js b/browser/base/content/test/tabs/browser_link_in_tab_title_and_url_prefilled_normal_page_other_target.js
index 7dc0e8fa45..53242ca359 100644
--- a/browser/base/content/test/tabs/browser_link_in_tab_title_and_url_prefilled_normal_page_other_target.js
+++ b/browser/base/content/test/tabs/browser_link_in_tab_title_and_url_prefilled_normal_page_other_target.js
@@ -45,7 +45,7 @@ add_task(async function normal_page__other_target__foreground__abort() {
       tab: BLANK_TITLE,
       urlbar: UrlbarTestUtils.trimURL(BLANK_URL),
     },
-    async actionWhileLoading(onTabLoaded) {
+    async actionWhileLoading() {
       info("Abort loading");
       document.getElementById("stop-button").click();
     },
@@ -117,7 +117,7 @@ add_task(async function normal_page__other_target__background__abort() {
       tab: WAIT_A_BIT_LOADING_TITLE,
       urlbar: UrlbarTestUtils.trimURL(HOME_URL),
     },
-    async actionWhileLoading(onTabLoaded) {
+    async actionWhileLoading() {
       info("Abort loading");
       document.getElementById("stop-button").click();
     },
diff --git a/browser/base/content/test/tabs/browser_long_data_url_label_truncation.js b/browser/base/content/test/tabs/browser_long_data_url_label_truncation.js
index db0571a2c0..89952b6c4d 100644
--- a/browser/base/content/test/tabs/browser_long_data_url_label_truncation.js
+++ b/browser/base/content/test/tabs/browser_long_data_url_label_truncation.js
@@ -33,7 +33,7 @@ add_task(async function test_ensure_truncation() {
 
   let fileReader = new FileReader();
   const DATA_URL = await new Promise(resolve => {
-    fileReader.addEventListener("load", e => resolve(fileReader.result));
+    fileReader.addEventListener("load", () => resolve(fileReader.result));
     fileReader.readAsDataURL(new Blob([MOBY], { type: "text/html" }));
   });
   // Substring the full URL to avoid log clutter because Assert will print
diff --git a/browser/base/content/test/tabs/browser_multiselect_tabs_move_to_new_window_contextmenu.js b/browser/base/content/test/tabs/browser_multiselect_tabs_move_to_new_window_contextmenu.js
index d668d21df8..f294769898 100644
--- a/browser/base/content/test/tabs/browser_multiselect_tabs_move_to_new_window_contextmenu.js
+++ b/browser/base/content/test/tabs/browser_multiselect_tabs_move_to_new_window_contextmenu.js
@@ -61,9 +61,9 @@ add_task(async function testLazyTabs() {
     await triggerClickOn(oldTabs[i], { ctrlKey: true });
   }
 
-  isnot(oldTabs[0].linkedPanel, "", `Old tab 0 shouldn't be lazy`);
+  isnot(oldTabs[0].linkedPanel, null, `Old tab 0 shouldn't be lazy`);
   for (let i = 1; i < numTabs; ++i) {
-    is(oldTabs[i].linkedPanel, "", `Old tab ${i} should be lazy`);
+    is(oldTabs[i].linkedPanel, null, `Old tab ${i} should be lazy`);
   }
 
   is(gBrowser.multiSelectedTabsCount, numTabs, `${numTabs} multiselected tabs`);
@@ -79,11 +79,11 @@ add_task(async function testLazyTabs() {
       if (i == 0) {
         isnot(
           oldTab.linkedPanel,
-          "",
+          null,
           `Old tab ${i} should continue not being lazy`
         );
       } else if (i > 0) {
-        is(oldTab.linkedPanel, "", `Old tab ${i} should continue being lazy`);
+        is(oldTab.linkedPanel, null, `Old tab ${i} should continue being lazy`);
       } else {
         return;
       }
@@ -101,9 +101,13 @@ add_task(async function testLazyTabs() {
   await tabsMoved;
   let newTabs = newWindow.gBrowser.tabs;
 
-  isnot(newTabs[0].linkedPanel, "", `New tab 0 should continue not being lazy`);
+  isnot(
+    newTabs[0].linkedPanel,
+    null,
+    `New tab 0 should continue not being lazy`
+  );
   for (let i = 1; i < numTabs; ++i) {
-    is(newTabs[i].linkedPanel, "", `New tab ${i} should continue being lazy`);
+    is(newTabs[i].linkedPanel, null, `New tab ${i} should continue being lazy`);
   }
 
   is(
diff --git a/browser/base/content/test/tabs/browser_new_tab_bookmarks_toolbar_height.js b/browser/base/content/test/tabs/browser_new_tab_bookmarks_toolbar_height.js
index 66258659fd..157254142d 100644
--- a/browser/base/content/test/tabs/browser_new_tab_bookmarks_toolbar_height.js
+++ b/browser/base/content/test/tabs/browser_new_tab_bookmarks_toolbar_height.js
@@ -12,7 +12,7 @@ async function expectHeightChanges(tab, expectedNewHeightChanges, msg) {
   let contentObservedHeightChanges = await ContentTask.spawn(
     tab.linkedBrowser,
     null,
-    async args => {
+    async () => {
       await new Promise(resolve => content.requestAnimationFrame(resolve));
       return content.document.body.innerText;
     }
@@ -109,7 +109,7 @@ add_task(async function () {
   info("Opening a new tab, making the previous tab non-selected");
   await expectBmToolbarVisibilityChange(
     () => {
-      BrowserOpenTab();
+      BrowserCommands.openTab();
       ok(
         !tab.selected,
         "non-new tab is in the background (not the selected tab)"
diff --git a/browser/base/content/test/tabs/browser_new_tab_in_privilegedabout_process_pref.js b/browser/base/content/test/tabs/browser_new_tab_in_privilegedabout_process_pref.js
index ec11951cb0..568510b20a 100644
--- a/browser/base/content/test/tabs/browser_new_tab_in_privilegedabout_process_pref.js
+++ b/browser/base/content/test/tabs/browser_new_tab_in_privilegedabout_process_pref.js
@@ -159,7 +159,7 @@ add_task(async function process_switching_through_navigation_features() {
       assertIsPrivilegedProcess(browser, "new tab opened from about:newtab");
 
       // Check that reload does not break the privileged about: content process affinity.
-      BrowserReload();
+      BrowserCommands.reload();
       await BrowserTestUtils.browserLoaded(browser, false, ABOUT_NEWTAB);
       assertIsPrivilegedProcess(browser, "about:newtab after reload");
 
diff --git a/browser/base/content/test/tabs/browser_new_tab_url.js b/browser/base/content/test/tabs/browser_new_tab_url.js
index 233cb4e59e..ab00560929 100644
--- a/browser/base/content/test/tabs/browser_new_tab_url.js
+++ b/browser/base/content/test/tabs/browser_new_tab_url.js
@@ -3,7 +3,7 @@
 "use strict";
 
 add_task(async function test_browser_open_newtab_default_url() {
-  BrowserOpenTab();
+  BrowserCommands.openTab();
   const tab = gBrowser.selectedTab;
 
   if (tab.linkedBrowser.currentURI.spec !== window.BROWSER_NEW_TAB_URL) {
@@ -19,7 +19,7 @@ add_task(async function test_browser_open_newtab_default_url() {
 add_task(async function test_browser_open_newtab_specific_url() {
   const url = "https://example.com";
 
-  BrowserOpenTab({ url });
+  BrowserCommands.openTab({ url });
   const tab = gBrowser.selectedTab;
 
   await BrowserTestUtils.browserLoaded(tab.linkedBrowser);
diff --git a/browser/base/content/test/tabs/browser_open_newtab_start_observer_notification.js b/browser/base/content/test/tabs/browser_open_newtab_start_observer_notification.js
index cb9fc3c6d7..2bc26cf667 100644
--- a/browser/base/content/test/tabs/browser_open_newtab_start_observer_notification.js
+++ b/browser/base/content/test/tabs/browser_open_newtab_start_observer_notification.js
@@ -9,10 +9,10 @@ add_task(async function test_browser_open_newtab_start_observer_notification() {
     Services.obs.addObserver(observe, "browser-open-newtab-start");
   });
 
-  // We're calling BrowserOpenTab() (rather the using BrowserTestUtils
+  // We're calling BrowserCommands.openTab() (rather the using BrowserTestUtils
   // because we want to be sure that it triggers the event to fire, since
   // it's very close to where various user-actions are triggered.
-  BrowserOpenTab();
+  BrowserCommands.openTab();
   const newTabCreatedPromise = await observerFiredPromise;
   const browser = await newTabCreatedPromise;
   const tab = gBrowser.selectedTab;
diff --git a/browser/base/content/test/tabs/browser_pinnedTabs_closeByKeyboard.js b/browser/base/content/test/tabs/browser_pinnedTabs_closeByKeyboard.js
index fbcd0bb492..4631afba42 100644
--- a/browser/base/content/test/tabs/browser_pinnedTabs_closeByKeyboard.js
+++ b/browser/base/content/test/tabs/browser_pinnedTabs_closeByKeyboard.js
@@ -5,14 +5,14 @@
 function test() {
   waitForExplicitFinish();
 
-  function testState(aPinned) {
+  function testState() {
     function elemAttr(id, attr) {
       return document.getElementById(id).getAttribute(attr);
     }
 
     is(
       elemAttr("key_close", "disabled"),
-      "",
+      null,
       "key_closed should always be enabled"
     );
     is(
diff --git a/browser/base/content/test/tabs/browser_privilegedmozilla_process_pref.js b/browser/base/content/test/tabs/browser_privilegedmozilla_process_pref.js
index 9e1c1ff5cd..922f94b07c 100644
--- a/browser/base/content/test/tabs/browser_privilegedmozilla_process_pref.js
+++ b/browser/base/content/test/tabs/browser_privilegedmozilla_process_pref.js
@@ -140,7 +140,7 @@ add_task(async function process_switching_through_navigation_features() {
     );
 
     // Check that reload does not break the privileged mozilla content process affinity.
-    BrowserReload();
+    BrowserCommands.reload();
     await BrowserTestUtils.browserLoaded(browser, false, TEST_HIGH1);
     is(
       browser.frameLoader.remoteTab.osPid,
diff --git a/browser/base/content/test/tabs/browser_removeTabs_order.js b/browser/base/content/test/tabs/browser_removeTabs_order.js
index 071cc03716..a993415653 100644
--- a/browser/base/content/test/tabs/browser_removeTabs_order.js
+++ b/browser/base/content/test/tabs/browser_removeTabs_order.js
@@ -16,7 +16,7 @@ add_task(async function () {
   // Add a beforeunload event listener in one of the tabs; it should be called
   // before closing any of the tabs.
   await ContentTask.spawn(tab2.linkedBrowser, null, async function () {
-    content.window.addEventListener("beforeunload", function (event) {}, true);
+    content.window.addEventListener("beforeunload", function () {}, true);
   });
 
   let permitUnloadSpy = sinon.spy(tab2.linkedBrowser, "asyncPermitUnload");
diff --git a/browser/base/content/test/tabs/browser_tab_label_picture_in_picture.js b/browser/base/content/test/tabs/browser_tab_label_picture_in_picture.js
index dae4ffc444..59cfd37c0d 100644
--- a/browser/base/content/test/tabs/browser_tab_label_picture_in_picture.js
+++ b/browser/base/content/test/tabs/browser_tab_label_picture_in_picture.js
@@ -11,7 +11,7 @@ add_task(async function test_pip_label_changes_tab() {
 
   let pipLabel = pipTab.querySelector(".tab-icon-sound-pip-label");
 
-  await BrowserTestUtils.withNewTab("about:blank", async browser => {
+  await BrowserTestUtils.withNewTab("about:blank", async () => {
     let selectedTab = newWin.document.querySelector(
       ".tabbrowser-tab[selected]"
     );
diff --git a/browser/base/content/test/tabs/browser_tab_manager_visibility.js b/browser/base/content/test/tabs/browser_tab_manager_visibility.js
index b7de777512..df6e75cd66 100644
--- a/browser/base/content/test/tabs/browser_tab_manager_visibility.js
+++ b/browser/base/content/test/tabs/browser_tab_manager_visibility.js
@@ -17,7 +17,7 @@ add_task(async function tab_manager_visibility_preference_on() {
       gBrowser: newWindow.gBrowser,
       url: TEST_HOSTNAME + DUMMY_PAGE_PATH,
     },
-    async function (browser) {
+    async function () {
       await Assert.ok(
         BrowserTestUtils.isVisible(
           newWindow.document.getElementById("alltabs-button")
@@ -39,7 +39,7 @@ add_task(async function tab_manager_visibility_preference_off() {
       gBrowser: newWindow.gBrowser,
       url: TEST_HOSTNAME + DUMMY_PAGE_PATH,
     },
-    async function (browser) {
+    async function () {
       await Assert.ok(
         BrowserTestUtils.isHidden(
           newWindow.document.getElementById("alltabs-button")
diff --git a/browser/base/content/test/tabs/browser_tab_preview.js b/browser/base/content/test/tabs/browser_tab_preview.js
index 718afbb940..0f83b1e28c 100644
--- a/browser/base/content/test/tabs/browser_tab_preview.js
+++ b/browser/base/content/test/tabs/browser_tab_preview.js
@@ -4,14 +4,14 @@
 
 "use strict";
 
+const { sinon } = ChromeUtils.importESModule(
+  "resource://testing-common/Sinon.sys.mjs"
+);
+
 async function openPreview(tab) {
-  const previewShown = BrowserTestUtils.waitForEvent(
-    document.getElementById("tabbrowser-tab-preview"),
-    "previewshown",
-    false,
-    e => {
-      return e.detail.tab === tab;
-    }
+  const previewShown = BrowserTestUtils.waitForPopupEvent(
+    document.getElementById("tab-preview-panel"),
+    "shown"
   );
   EventUtils.synthesizeMouseAtCenter(tab, { type: "mouseover" });
   return previewShown;
@@ -19,9 +19,9 @@ async function openPreview(tab) {
 
 async function closePreviews() {
   const tabs = document.getElementById("tabbrowser-tabs");
-  const previewHidden = BrowserTestUtils.waitForEvent(
-    document.getElementById("tabbrowser-tab-preview"),
-    "previewhidden"
+  const previewHidden = BrowserTestUtils.waitForPopupEvent(
+    document.getElementById("tab-preview-panel"),
+    "hidden"
   );
   EventUtils.synthesizeMouse(tabs, 0, tabs.outerHeight + 1, {
     type: "mouseout",
@@ -53,35 +53,24 @@ add_task(async function hoverTests() {
   const tabUrl2 =
     "data:text/html,<html><head><title>Second New Tab</title></head><body>Hello</body></html>";
   const tab2 = await BrowserTestUtils.openNewForegroundTab(gBrowser, tabUrl2);
-  const previewContainer = document.getElementById("tabbrowser-tab-preview");
+  const previewContainer = document.getElementById("tab-preview-panel");
 
   await openPreview(tab1);
-  Assert.ok(
-    ["open", "showing"].includes(previewContainer.panel.state),
-    "tab1 preview shown"
-  );
   Assert.equal(
-    previewContainer.renderRoot.querySelector(".tab-preview-title").innerText,
+    previewContainer.querySelector(".tab-preview-title").innerText,
     "First New Tab",
     "Preview of tab1 shows correct title"
   );
 
+  await closePreviews();
   await openPreview(tab2);
-  Assert.ok(
-    ["open", "showing"].includes(previewContainer.panel.state),
-    "tab2 preview shown"
-  );
   Assert.equal(
-    previewContainer.renderRoot.querySelector(".tab-preview-title").innerText,
+    previewContainer.querySelector(".tab-preview-title").innerText,
     "Second New Tab",
     "Preview of tab2 shows correct title"
   );
 
   await closePreviews();
-  Assert.ok(
-    ["closed", "hiding"].includes(previewContainer.panel.state),
-    "preview container is now hidden"
-  );
 
   BrowserTestUtils.removeTab(tab1);
   BrowserTestUtils.removeTab(tab2);
@@ -105,29 +94,41 @@ add_task(async function thumbnailTests() {
   const tab1 = await BrowserTestUtils.openNewForegroundTab(gBrowser, tabUrl1);
   const tabUrl2 = "about:blank";
   const tab2 = await BrowserTestUtils.openNewForegroundTab(gBrowser, tabUrl2);
-  const previewContainer = document.getElementById("tabbrowser-tab-preview");
+  const previewPanel = document.getElementById("tab-preview-panel");
 
-  const thumbnailUpdated = BrowserTestUtils.waitForEvent(
-    previewContainer,
-    "previewThumbnailUpdated"
+  let thumbnailUpdated = BrowserTestUtils.waitForEvent(
+    previewPanel,
+    "previewThumbnailUpdated",
+    false,
+    evt => evt.detail.thumbnail
   );
   await openPreview(tab1);
   await thumbnailUpdated;
   Assert.ok(
-    previewContainer.renderRoot.querySelectorAll("img,canvas").length,
+    previewPanel.querySelectorAll(
+      ".tab-preview-thumbnail-container img, .tab-preview-thumbnail-container canvas"
+    ).length,
     "Tab1 preview contains thumbnail"
   );
 
+  await closePreviews();
+  thumbnailUpdated = BrowserTestUtils.waitForEvent(
+    previewPanel,
+    "previewThumbnailUpdated"
+  );
   await openPreview(tab2);
+  await thumbnailUpdated;
   Assert.equal(
-    previewContainer.renderRoot.querySelectorAll("img,canvas").length,
+    previewPanel.querySelectorAll(
+      ".tab-preview-thumbnail-container img, .tab-preview-thumbnail-container canvas"
+    ).length,
     0,
     "Tab2 (selected) does not contain thumbnail"
   );
 
-  const previewHidden = BrowserTestUtils.waitForEvent(
-    document.getElementById("tabbrowser-tab-preview"),
-    "previewhidden"
+  const previewHidden = BrowserTestUtils.waitForPopupEvent(
+    previewPanel,
+    "hidden"
   );
 
   BrowserTestUtils.removeTab(tab1);
@@ -143,6 +144,102 @@ add_task(async function thumbnailTests() {
   });
 });
 
+/**
+ * make sure delay is applied when mouse leaves tabstrip
+ * but not when moving between tabs on the tabstrip
+ */
+add_task(async function delayTests() {
+  const tabUrl1 =
+    "data:text/html,<html><head><title>First New Tab</title></head><body>Hello</body></html>";
+  const tab1 = await BrowserTestUtils.openNewForegroundTab(gBrowser, tabUrl1);
+  const tabUrl2 =
+    "data:text/html,<html><head><title>Second New Tab</title></head><body>Hello</body></html>";
+  const tab2 = await BrowserTestUtils.openNewForegroundTab(gBrowser, tabUrl2);
+  const previewComponent = gBrowser.tabContainer.previewPanel;
+  const previewElement = document.getElementById("tab-preview-panel");
+
+  sinon.spy(previewComponent, "deactivate");
+
+  await openPreview(tab1);
+
+  // I can't fake this like in hoverTests, need to send an updated-tab signal
+  //await openPreview(tab2);
+
+  const previewHidden = BrowserTestUtils.waitForPopupEvent(
+    previewElement,
+    "hidden"
+  );
+  Assert.ok(
+    !previewComponent.deactivate.called,
+    "Delay is not reset when moving between tabs"
+  );
+
+  EventUtils.synthesizeMouseAtCenter(document.getElementById("reload-button"), {
+    type: "mousemove",
+  });
+
+  await previewHidden;
+
+  Assert.ok(
+    previewComponent.deactivate.called,
+    "Delay is reset when cursor leaves tabstrip"
+  );
+
+  BrowserTestUtils.removeTab(tab1);
+  BrowserTestUtils.removeTab(tab2);
+  sinon.restore();
+});
+
+/**
+ * Dragging a tab should deactivate the preview
+ */
+add_task(async function dragTests() {
+  await SpecialPowers.pushPrefEnv({
+    set: [["ui.tooltip.delay_ms", 1000]],
+  });
+  const tabUrl1 =
+    "data:text/html,<html><head><title>First New Tab</title></head><body>Hello</body></html>";
+  const tab1 = await BrowserTestUtils.openNewForegroundTab(gBrowser, tabUrl1);
+  const tabUrl2 =
+    "data:text/html,<html><head><title>Second New Tab</title></head><body>Hello</body></html>";
+  const tab2 = await BrowserTestUtils.openNewForegroundTab(gBrowser, tabUrl2);
+  const previewComponent = gBrowser.tabContainer.previewPanel;
+  const previewElement = document.getElementById("tab-preview-panel");
+
+  sinon.spy(previewComponent, "deactivate");
+
+  await openPreview(tab1);
+  const previewHidden = BrowserTestUtils.waitForPopupEvent(
+    previewElement,
+    "hidden"
+  );
+  let dragend = BrowserTestUtils.waitForEvent(tab1, "dragend");
+  EventUtils.synthesizePlainDragAndDrop({
+    srcElement: tab1,
+    destElement: tab2,
+  });
+
+  await previewHidden;
+
+  Assert.ok(
+    previewComponent.deactivate.called,
+    "delay is reset after drag started"
+  );
+
+  await dragend;
+
+  BrowserTestUtils.removeTab(tab1);
+  BrowserTestUtils.removeTab(tab2);
+  sinon.restore();
+
+  // Move the mouse outside of the tab strip.
+  EventUtils.synthesizeMouseAtCenter(document.documentElement, {
+    type: "mouseover",
+  });
+
+  await SpecialPowers.popPrefEnv();
+});
+
 /**
  * Wheel events at the document-level of the window should hide the preview.
  */
@@ -155,9 +252,9 @@ add_task(async function wheelTests() {
   await openPreview(tab1);
 
   const tabs = document.getElementById("tabbrowser-tabs");
-  const previewHidden = BrowserTestUtils.waitForEvent(
-    document.getElementById("tabbrowser-tab-preview"),
-    "previewhidden"
+  const previewHidden = BrowserTestUtils.waitForPopupEvent(
+    document.getElementById("tab-preview-panel"),
+    "hidden"
   );
 
   // Copied from apz_test_native_event_utils.js
diff --git a/browser/base/content/test/tabs/browser_tab_tooltips.js b/browser/base/content/test/tabs/browser_tab_tooltips.js
index ee82816bce..79be4d0a36 100644
--- a/browser/base/content/test/tabs/browser_tab_tooltips.js
+++ b/browser/base/content/test/tabs/browser_tab_tooltips.js
@@ -57,7 +57,7 @@ add_task(async function () {
   );
   is(
     tooltip.getAttribute("position"),
-    "",
+    null,
     "tooltip position attribute for tab"
   );
 
diff --git a/browser/base/content/test/tabs/browser_tabswitch_select.js b/browser/base/content/test/tabs/browser_tabswitch_select.js
index 3868764bed..b22a75c79c 100644
--- a/browser/base/content/test/tabs/browser_tabswitch_select.js
+++ b/browser/base/content/test/tabs/browser_tabswitch_select.js
@@ -35,7 +35,7 @@ add_task(async function () {
   let fullScreenEntered = TestUtils.waitForCondition(
     () => document.documentElement.getAttribute("sizemode") == "fullscreen"
   );
-  BrowserFullScreen();
+  BrowserCommands.fullScreen();
   await fullScreenEntered;
 
   tab2.linkedBrowser.focus();
@@ -54,7 +54,7 @@ add_task(async function () {
   let fullScreenExited = TestUtils.waitForCondition(
     () => document.documentElement.getAttribute("sizemode") != "fullscreen"
   );
-  BrowserFullScreen();
+  BrowserCommands.fullScreen();
   await fullScreenExited;
 
   BrowserTestUtils.removeTab(gBrowser.selectedTab);
diff --git a/browser/base/content/test/tabs/browser_tabswitch_updatecommands.js b/browser/base/content/test/tabs/browser_tabswitch_updatecommands.js
index b5d2762eec..82f9eb871b 100644
--- a/browser/base/content/test/tabs/browser_tabswitch_updatecommands.js
+++ b/browser/base/content/test/tabs/browser_tabswitch_updatecommands.js
@@ -8,7 +8,7 @@ add_task(async function () {
   let tab2 = await BrowserTestUtils.openNewForegroundTab(gBrowser, uri);
 
   let updates = [];
-  function countUpdates(event) {
+  function countUpdates() {
     updates.push(new Error().stack);
   }
   let updater = document.getElementById("editMenuCommandSetAll");
diff --git a/browser/base/content/test/tabs/browser_viewsource_of_data_URI_in_file_process.js b/browser/base/content/test/tabs/browser_viewsource_of_data_URI_in_file_process.js
index b5ae94ce84..2c301a400d 100644
--- a/browser/base/content/test/tabs/browser_viewsource_of_data_URI_in_file_process.js
+++ b/browser/base/content/test/tabs/browser_viewsource_of_data_URI_in_file_process.js
@@ -33,7 +33,7 @@ add_task(async function () {
 
     // Make sure we can view-source on the data URI page.
     let promiseTab = BrowserTestUtils.waitForNewTab(gBrowser, DATA_URI_SOURCE);
-    BrowserViewSource(fileBrowser);
+    BrowserCommands.viewSource(fileBrowser);
     let viewSourceTab = await promiseTab;
     registerCleanupFunction(async function () {
       BrowserTestUtils.removeTab(viewSourceTab);
diff --git a/browser/base/content/test/tabs/browser_window_open_modifiers.js b/browser/base/content/test/tabs/browser_window_open_modifiers.js
index b4376d6824..2ef951efef 100644
--- a/browser/base/content/test/tabs/browser_window_open_modifiers.js
+++ b/browser/base/content/test/tabs/browser_window_open_modifiers.js
@@ -97,7 +97,7 @@ add_task(async function () {
           BrowserTestUtils.synthesizeMouseAtCenter(id, { ...event }, browser);
         } else {
           // Make sure the keyboard activates a simple button on the page.
-          await ContentTask.spawn(browser, id, elementId => {
+          await ContentTask.spawn(browser, id, () => {
             content.document.querySelector("#focus-result").value = "";
             content.document.querySelector("#focus-check").focus();
           });
diff --git a/browser/base/content/test/tabs/common_link_in_tab_title_and_url_prefilled.js b/browser/base/content/test/tabs/common_link_in_tab_title_and_url_prefilled.js
index a06b982615..bff14e5ced 100644
--- a/browser/base/content/test/tabs/common_link_in_tab_title_and_url_prefilled.js
+++ b/browser/base/content/test/tabs/common_link_in_tab_title_and_url_prefilled.js
@@ -244,7 +244,7 @@ async function synthesizeMouse(browser, link, event) {
 async function waitForNewTabWithLoadRequest() {
   return new Promise(resolve =>
     gBrowser.addTabsProgressListener({
-      onStateChange(aBrowser, aWebProgress, aRequest, aStateFlags, aStatus) {
+      onStateChange(aBrowser, aWebProgress, aRequest, aStateFlags) {
         if (aStateFlags & Ci.nsIWebProgressListener.STATE_START) {
           gBrowser.removeTabsProgressListener(this);
           resolve(gBrowser.getTabForBrowser(aBrowser));
diff --git a/browser/base/content/test/webextensions/browser_extension_update_background.js b/browser/base/content/test/webextensions/browser_extension_update_background.js
index 490544b2ec..5619bacb4d 100644
--- a/browser/base/content/test/webextensions/browser_extension_update_background.js
+++ b/browser/base/content/test/webextensions/browser_extension_update_background.js
@@ -87,7 +87,7 @@ async function backgroundUpdateTest(url, id, checkIconFn) {
   let addonId = addon.id;
 
   ok(addon, "Addon was installed");
-  is(getBadgeStatus(), "", "Should not start out with an addon alert badge");
+  is(getBadgeStatus(), null, "Should not start out with an addon alert badge");
 
   // Trigger an update check and wait for the update for this addon
   // to be downloaded.
@@ -156,7 +156,7 @@ async function backgroundUpdateTest(url, id, checkIconFn) {
   BrowserTestUtils.removeTab(tab);
 
   // Alert badge and hamburger menu items should be gone
-  is(getBadgeStatus(), "", "Addon alert badge should be gone");
+  is(getBadgeStatus(), null, "Addon alert badge should be gone");
 
   await gCUITestUtils.openMainMenu();
   addons = PanelUI.addonNotificationContainer;
@@ -205,7 +205,7 @@ async function backgroundUpdateTest(url, id, checkIconFn) {
 
   BrowserTestUtils.removeTab(tab);
 
-  is(getBadgeStatus(), "", "Addon alert badge should be gone");
+  is(getBadgeStatus(), null, "Addon alert badge should be gone");
 
   await addon.uninstall();
   await SpecialPowers.popPrefEnv();
diff --git a/browser/base/content/test/webextensions/browser_extension_update_background_noprompt.js b/browser/base/content/test/webextensions/browser_extension_update_background_noprompt.js
index a0b10c82e2..204e7fb44a 100644
--- a/browser/base/content/test/webextensions/browser_extension_update_background_noprompt.js
+++ b/browser/base/content/test/webextensions/browser_extension_update_background_noprompt.js
@@ -81,7 +81,7 @@ async function testNoPrompt(origUrl, id) {
   await updatePromise;
 
   // There should be no notifications about the update
-  is(getBadgeStatus(), "", "Should not have addon alert badge");
+  is(getBadgeStatus(), null, "Should not have addon alert badge");
 
   await gCUITestUtils.openMainMenu();
   let addons = PanelUI.addonNotificationContainer;
diff --git a/browser/base/content/test/webextensions/browser_legacy_webext.xpi b/browser/base/content/test/webextensions/browser_legacy_webext.xpi
index a3bdf6f832..afd0a8bcee 100644
Binary files a/browser/base/content/test/webextensions/browser_legacy_webext.xpi and b/browser/base/content/test/webextensions/browser_legacy_webext.xpi differ
diff --git a/browser/base/content/test/webextensions/browser_permissions_installTrigger.js b/browser/base/content/test/webextensions/browser_permissions_installTrigger.js
index a227518ebb..36b4efff8b 100644
--- a/browser/base/content/test/webextensions/browser_permissions_installTrigger.js
+++ b/browser/base/content/test/webextensions/browser_permissions_installTrigger.js
@@ -9,6 +9,12 @@ async function installTrigger(filename) {
       ["extensions.InstallTriggerImpl.enabled", true],
       // Relax the user input requirements while running this test.
       ["xpinstall.userActivation.required", false],
+      // This test asserts that the extension icon is in the install dialog
+      // and so it requires the signature checks to be enabled (otherwise the
+      // extension icon is expected to be replaced by a warning icon) and the
+      // two test extension used by this test (browser_webext_nopermissions.xpi
+      // and browser_webext_permissions.xpi) are signed using AMO stage signatures.
+      ["xpinstall.signatures.dev-root", true],
     ],
   });
   BrowserTestUtils.startLoadingURIString(
diff --git a/browser/base/content/test/webextensions/browser_permissions_local_file.js b/browser/base/content/test/webextensions/browser_permissions_local_file.js
index 7f8f256e14..22dff8cb38 100644
--- a/browser/base/content/test/webextensions/browser_permissions_local_file.js
+++ b/browser/base/content/test/webextensions/browser_permissions_local_file.js
@@ -32,9 +32,22 @@ add_task(async function test_install_extension_from_local_file() {
     },
   });
 
+  await SpecialPowers.pushPrefEnv({
+    set: [
+      // This test asserts that the extension icon is in the install dialog
+      // and so it requires the signature checks to be enabled (otherwise the
+      // extension icon is expected to be replaced by a warning icon) and the
+      // two test extension used by this test (browser_webext_nopermissions.xpi
+      // and browser_webext_permissions.xpi) are signed using AMO stage signatures.
+      ["xpinstall.signatures.dev-root", true],
+    ],
+  });
+
   // Install the add-ons.
   await testInstallMethod(installFile, "installLocal");
 
+  await SpecialPowers.popPrefEnv();
+
   // Check we got an installId.
   ok(
     firstInstallId != null && !isNaN(firstInstallId),
diff --git a/browser/base/content/test/webextensions/browser_permissions_mozAddonManager.js b/browser/base/content/test/webextensions/browser_permissions_mozAddonManager.js
index 55a578221d..d54038bffe 100644
--- a/browser/base/content/test/webextensions/browser_permissions_mozAddonManager.js
+++ b/browser/base/content/test/webextensions/browser_permissions_mozAddonManager.js
@@ -9,6 +9,17 @@ async function installMozAM(filename) {
   );
   await BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser);
 
+  await SpecialPowers.pushPrefEnv({
+    set: [
+      // This test asserts that the extension icon is in the install dialog
+      // and so it requires the signature checks to be enabled (otherwise the
+      // extension icon is expected to be replaced by a warning icon) and the
+      // two test extension used by this test (browser_webext_nopermissions.xpi
+      // and browser_webext_permissions.xpi) are signed using AMO stage signatures.
+      ["xpinstall.signatures.dev-root", true],
+    ],
+  });
+
   await SpecialPowers.spawn(
     gBrowser.selectedBrowser,
     [`${BASE}/${filename}`],
@@ -16,6 +27,8 @@ async function installMozAM(filename) {
       await content.wrappedJSObject.installMozAM(url);
     }
   );
+
+  await SpecialPowers.popPrefEnv();
 }
 
 add_task(() => testInstallMethod(installMozAM, "installAmo"));
diff --git a/browser/base/content/test/webextensions/browser_permissions_pointerevent.js b/browser/base/content/test/webextensions/browser_permissions_pointerevent.js
index 188aa8e3bf..2809ffe9b4 100644
--- a/browser/base/content/test/webextensions/browser_permissions_pointerevent.js
+++ b/browser/base/content/test/webextensions/browser_permissions_pointerevent.js
@@ -9,15 +9,15 @@ add_task(async function test_pointerevent() {
       e.preventDefault();
     });
 
-    document.addEventListener("mousedown", e => {
+    document.addEventListener("mousedown", () => {
       browser.test.assertTrue(true, "Should receive mousedown");
     });
 
-    document.addEventListener("mouseup", e => {
+    document.addEventListener("mouseup", () => {
       browser.test.assertTrue(true, "Should receive mouseup");
     });
 
-    document.addEventListener("pointerup", e => {
+    document.addEventListener("pointerup", () => {
       browser.test.assertTrue(true, "Should receive pointerup");
       browser.test.sendMessage("done");
     });
diff --git a/browser/base/content/test/webextensions/browser_update_checkForUpdates.js b/browser/base/content/test/webextensions/browser_update_checkForUpdates.js
index b902527cae..c9e59556e1 100644
--- a/browser/base/content/test/webextensions/browser_update_checkForUpdates.js
+++ b/browser/base/content/test/webextensions/browser_update_checkForUpdates.js
@@ -3,7 +3,7 @@ function checkAll(win) {
   triggerPageOptionsAction(win, "check-for-updates");
   return new Promise(resolve => {
     let observer = {
-      observe(subject, topic, data) {
+      observe() {
         Services.obs.removeObserver(observer, "EM-update-check-finished");
         resolve();
       },
diff --git a/browser/base/content/test/webextensions/browser_webext_nopermissions.xpi b/browser/base/content/test/webextensions/browser_webext_nopermissions.xpi
index ab97d96a11..87500ceb38 100644
Binary files a/browser/base/content/test/webextensions/browser_webext_nopermissions.xpi and b/browser/base/content/test/webextensions/browser_webext_nopermissions.xpi differ
diff --git a/browser/base/content/test/webextensions/browser_webext_permissions.xpi b/browser/base/content/test/webextensions/browser_webext_permissions.xpi
index a8c8c38ef8..8149ce7b6b 100644
Binary files a/browser/base/content/test/webextensions/browser_webext_permissions.xpi and b/browser/base/content/test/webextensions/browser_webext_permissions.xpi differ
diff --git a/browser/base/content/test/webextensions/browser_webext_update1.xpi b/browser/base/content/test/webextensions/browser_webext_update1.xpi
index 086b3839b9..66ad3e1b31 100644
Binary files a/browser/base/content/test/webextensions/browser_webext_update1.xpi and b/browser/base/content/test/webextensions/browser_webext_update1.xpi differ
diff --git a/browser/base/content/test/webextensions/browser_webext_update2.xpi b/browser/base/content/test/webextensions/browser_webext_update2.xpi
index 19967c39c0..a120a64c6d 100644
Binary files a/browser/base/content/test/webextensions/browser_webext_update2.xpi and b/browser/base/content/test/webextensions/browser_webext_update2.xpi differ
diff --git a/browser/base/content/test/webextensions/browser_webext_update_icon1.xpi b/browser/base/content/test/webextensions/browser_webext_update_icon1.xpi
index 24cb7616d2..040f8f8c97 100644
Binary files a/browser/base/content/test/webextensions/browser_webext_update_icon1.xpi and b/browser/base/content/test/webextensions/browser_webext_update_icon1.xpi differ
diff --git a/browser/base/content/test/webextensions/browser_webext_update_icon2.xpi b/browser/base/content/test/webextensions/browser_webext_update_icon2.xpi
index fd9cf7eb0e..0b13e7c7dd 100644
Binary files a/browser/base/content/test/webextensions/browser_webext_update_icon2.xpi and b/browser/base/content/test/webextensions/browser_webext_update_icon2.xpi differ
diff --git a/browser/base/content/test/webextensions/browser_webext_update_perms1.xpi b/browser/base/content/test/webextensions/browser_webext_update_perms1.xpi
index f4942f9082..60b6643a12 100644
Binary files a/browser/base/content/test/webextensions/browser_webext_update_perms1.xpi and b/browser/base/content/test/webextensions/browser_webext_update_perms1.xpi differ
diff --git a/browser/base/content/test/webextensions/browser_webext_update_perms2.xpi b/browser/base/content/test/webextensions/browser_webext_update_perms2.xpi
index 2c023edc9d..64c2afb473 100644
Binary files a/browser/base/content/test/webextensions/browser_webext_update_perms2.xpi and b/browser/base/content/test/webextensions/browser_webext_update_perms2.xpi differ
diff --git a/browser/base/content/test/webextensions/head.js b/browser/base/content/test/webextensions/head.js
index 84f7cd02d7..f1183c61b8 100644
--- a/browser/base/content/test/webextensions/head.js
+++ b/browser/base/content/test/webextensions/head.js
@@ -302,7 +302,7 @@ function checkNotification(panel, checkIcon, permissions, sideloaded) {
  *
  * @returns {Promise}
  */
-async function testInstallMethod(installFn, telemetryBase) {
+async function testInstallMethod(installFn) {
   const PERMS_XPI = "browser_webext_permissions.xpi";
   const NO_PERMS_XPI = "browser_webext_nopermissions.xpi";
   const ID = "permissions@test.mozilla.org";
diff --git a/browser/base/content/test/webrtc/browser_devices_get_user_media_anim.js b/browser/base/content/test/webrtc/browser_devices_get_user_media_anim.js
index dd20a672c3..f1052565b8 100644
--- a/browser/base/content/test/webrtc/browser_devices_get_user_media_anim.js
+++ b/browser/base/content/test/webrtc/browser_devices_get_user_media_anim.js
@@ -58,7 +58,7 @@ var gTests = [
         );
         is(
           gBrowser.selectedTab.getAttribute("sharing"),
-          "",
+          null,
           "the new tab doesn't have the 'sharing' attribute"
         );
         is(
@@ -89,7 +89,7 @@ var gTests = [
         await TestUtils.waitForCondition(() => !tab.getAttribute("sharing"));
         is(
           tab.getAttribute("sharing"),
-          "",
+          null,
           "the tab no longer has the 'sharing' attribute after closing the stream"
         );
       }
diff --git a/browser/base/content/test/webrtc/browser_devices_get_user_media_by_device_id.js b/browser/base/content/test/webrtc/browser_devices_get_user_media_by_device_id.js
index 3e5ca0668a..9598bb565c 100644
--- a/browser/base/content/test/webrtc/browser_devices_get_user_media_by_device_id.js
+++ b/browser/base/content/test/webrtc/browser_devices_get_user_media_by_device_id.js
@@ -49,7 +49,7 @@ add_task(async function test_get_user_media_by_device_id() {
     .filter(d => d.kind == "videoinput")
     .map(d => d.deviceId)[0];
 
-  await BrowserTestUtils.withNewTab(TEST_PAGE, async browser => {
+  await BrowserTestUtils.withNewTab(TEST_PAGE, async () => {
     let promise = promisePopupNotificationShown("webRTC-shareDevices");
     let observerPromise = expectObserverCalled("getUserMedia:request");
     await promiseRequestDevice({ deviceId: { exact: audioId } });
diff --git a/browser/base/content/test/webrtc/browser_devices_get_user_media_grace.js b/browser/base/content/test/webrtc/browser_devices_get_user_media_grace.js
index 0df69bb9da..624bc07ce0 100644
--- a/browser/base/content/test/webrtc/browser_devices_get_user_media_grace.js
+++ b/browser/base/content/test/webrtc/browser_devices_get_user_media_grace.js
@@ -171,7 +171,7 @@ var gTests = [
 
   {
     desc: "getUserMedia camera+mic survives page reload but not past grace",
-    run: async function checkAudioVideoGracePastReload(browser) {
+    run: async function checkAudioVideoGracePastReload() {
       await prompt(true, true);
       await allow(true, true);
       await closeStream();
@@ -240,7 +240,7 @@ var gTests = [
 
       info("Open same page in a new tab");
       await disableObserverVerification();
-      await BrowserTestUtils.withNewTab(SAME_ORIGIN + PATH, async browser => {
+      await BrowserTestUtils.withNewTab(SAME_ORIGIN + PATH, async () => {
         info("In new tab, gUM(camera+mic) causes a prompt.");
         await prompt(true, true);
       });
@@ -329,7 +329,7 @@ var gTests = [
 
   {
     desc: "getUserMedia camera+mic grace period cleared on permission block",
-    run: async function checkAudioVideoGraceEndsNewTab(browser) {
+    run: async function checkAudioVideoGraceEndsNewTab() {
       await SpecialPowers.pushPrefEnv({
         set: [["privacy.webrtc.deviceGracePeriodTimeoutMs", 10000]],
       });
diff --git a/browser/base/content/test/webrtc/browser_devices_select_audio_output.js b/browser/base/content/test/webrtc/browser_devices_select_audio_output.js
index fbced1f5cc..df75f39a1a 100644
--- a/browser/base/content/test/webrtc/browser_devices_select_audio_output.js
+++ b/browser/base/content/test/webrtc/browser_devices_select_audio_output.js
@@ -220,7 +220,7 @@ var gTests = [
         gBrowser.selectedBrowser.browsingContext,
         "getUserMedia:response:allow",
         1,
-        (aSubject, aTopic, aData) => {
+        aSubject => {
           const device = aSubject
             .QueryInterface(Ci.nsIArrayExtensions)
             .GetElementAt(0).wrappedJSObject;
diff --git a/browser/base/content/test/webrtc/browser_webrtc_hooks.js b/browser/base/content/test/webrtc/browser_webrtc_hooks.js
index e980b15286..3fb2fc9f8d 100644
--- a/browser/base/content/test/webrtc/browser_webrtc_hooks.js
+++ b/browser/base/content/test/webrtc/browser_webrtc_hooks.js
@@ -122,7 +122,7 @@ var gTests = [
     run: async function testDeferredBlocker(browser) {
       Events.on();
 
-      let blocker = params => Promise.resolve("allow");
+      let blocker = () => Promise.resolve("allow");
       webrtcUI.addPeerConnectionBlocker(blocker);
 
       await tryPeerConnection(browser);
@@ -138,7 +138,7 @@ var gTests = [
     run: async function testBlockerDeny(browser) {
       Events.on();
 
-      let blocker = params => "deny";
+      let blocker = () => "deny";
       webrtcUI.addPeerConnectionBlocker(blocker);
 
       await tryPeerConnection(browser, "NotAllowedError");
@@ -156,14 +156,14 @@ var gTests = [
       Events.on();
 
       let blocker1Called = false,
-        blocker1 = params => {
+        blocker1 = () => {
           blocker1Called = true;
           return "allow";
         };
       webrtcUI.addPeerConnectionBlocker(blocker1);
 
       let blocker2Called = false,
-        blocker2 = params => {
+        blocker2 = () => {
           blocker2Called = true;
           return "allow";
         };
@@ -187,14 +187,14 @@ var gTests = [
       Events.on();
 
       let blocker1Called = false,
-        blocker1 = params => {
+        blocker1 = () => {
           blocker1Called = true;
           return "allow";
         };
       webrtcUI.addPeerConnectionBlocker(blocker1);
 
       let blocker2Called = false,
-        blocker2 = params => {
+        blocker2 = () => {
           blocker2Called = true;
           return "deny";
         };
@@ -218,14 +218,14 @@ var gTests = [
       Events.on();
 
       let blocker1Called = false,
-        blocker1 = params => {
+        blocker1 = () => {
           blocker1Called = true;
           return "deny";
         };
       webrtcUI.addPeerConnectionBlocker(blocker1);
 
       let blocker2Called = false,
-        blocker2 = params => {
+        blocker2 = () => {
           blocker2Called = true;
           return "allow";
         };
@@ -252,14 +252,14 @@ var gTests = [
       Events.on();
 
       let blocker1Called = false,
-        blocker1 = params => {
+        blocker1 = () => {
           blocker1Called = true;
           return "allow";
         };
       webrtcUI.addPeerConnectionBlocker(blocker1);
 
       let blocker2Called = false,
-        blocker2 = params => {
+        blocker2 = () => {
           blocker2Called = true;
           return "allow";
         };
@@ -283,14 +283,14 @@ var gTests = [
     run: async function testBlockerThrows(browser) {
       Events.on();
       let blocker1Called = false,
-        blocker1 = params => {
+        blocker1 = () => {
           blocker1Called = true;
           throw new Error("kaboom");
         };
       webrtcUI.addPeerConnectionBlocker(blocker1);
 
       let blocker2Called = false,
-        blocker2 = params => {
+        blocker2 = () => {
           blocker2Called = true;
           return "allow";
         };
@@ -313,10 +313,10 @@ var gTests = [
     run: async function testBlockerCancel(browser) {
       let blocker,
         blockerPromise = new Promise(resolve => {
-          blocker = params => {
+          blocker = () => {
             resolve();
             // defer indefinitely
-            return new Promise(innerResolve => {});
+            return new Promise(() => {});
           };
         });
       webrtcUI.addPeerConnectionBlocker(blocker);
diff --git a/browser/base/content/test/webrtc/head.js b/browser/base/content/test/webrtc/head.js
index 639ae2e51a..694875bd21 100644
--- a/browser/base/content/test/webrtc/head.js
+++ b/browser/base/content/test/webrtc/head.js
@@ -146,7 +146,7 @@ async function assertWebRTCIndicatorStatus(expected) {
   if (!expected) {
     let win = Services.wm.getMostRecentWindow("Browser:WebRTCGlobalIndicator");
     if (win) {
-      await new Promise((resolve, reject) => {
+      await new Promise(resolve => {
         win.addEventListener("unload", function listener(e) {
           if (e.target == win.document) {
             win.removeEventListener("unload", listener);
@@ -308,7 +308,7 @@ function expectObserverCalledOnClose(
       {
         topic: aTopic,
         count: 1,
-        filterFunctionSource: ((subject, topic, data) => {
+        filterFunctionSource: ((subject, topic) => {
           Services.cpmm.sendAsyncMessage("WebRTCTest:ObserverCalled", {
             topic,
           });
@@ -1061,7 +1061,7 @@ async function promiseReloadFrame(aFrameId, aBrowsingContext) {
   let loadedPromise = BrowserTestUtils.browserLoaded(
     gBrowser.selectedBrowser,
     true,
-    arg => {
+    () => {
       return true;
     }
   );
diff --git a/browser/base/content/test/zoom/browser.toml b/browser/base/content/test/zoom/browser.toml
index 281fb9329c..e2aa0c077a 100644
--- a/browser/base/content/test/zoom/browser.toml
+++ b/browser/base/content/test/zoom/browser.toml
@@ -34,7 +34,7 @@ https_first_disabled = true
 
 ["browser_sitespecific_video_zoom.js"]
 https_first_disabled = true
-support-files = ["../general/video.ogg"]
+support-files = ["../general/video.webm"]
 skip-if = [
   "os == 'win' && debug", # Bug 1315042
   "verify && debug && os == 'linux'", # Bug 1315042
diff --git a/browser/base/content/test/zoom/browser_sitespecific_video_zoom.js b/browser/base/content/test/zoom/browser_sitespecific_video_zoom.js
index 589e3d09cf..94fd0dee56 100644
--- a/browser/base/content/test/zoom/browser_sitespecific_video_zoom.js
+++ b/browser/base/content/test/zoom/browser_sitespecific_video_zoom.js
@@ -8,7 +8,7 @@ const TEST_PAGE =
   "http://example.org/browser/browser/base/content/test/zoom/zoom_test.html";
 const TEST_VIDEO =
   // eslint-disable-next-line @microsoft/sdl/no-insecure-url
-  "http://example.org/browser/browser/base/content/test/general/video.ogg";
+  "http://example.org/browser/browser/base/content/test/general/video.webm";
 
 var gTab1, gTab2, gLevel1;
 
diff --git a/browser/base/content/test/zoom/browser_zoom_commands.js b/browser/base/content/test/zoom/browser_zoom_commands.js
index 88b6f42059..ef49a5794e 100644
--- a/browser/base/content/test/zoom/browser_zoom_commands.js
+++ b/browser/base/content/test/zoom/browser_zoom_commands.js
@@ -65,7 +65,7 @@ function assertTextZoomCommandCheckedState(isChecked) {
  * zoom levels and/or preferences on an individual browser.
  */
 add_task(async function test_update_browser_zoom() {
-  await BrowserTestUtils.withNewTab(TEST_PAGE_URL, async browser => {
+  await BrowserTestUtils.withNewTab(TEST_PAGE_URL, async () => {
     let currentZoom = await FullZoomHelper.getGlobalValue();
     Assert.equal(
       currentZoom,
@@ -136,7 +136,7 @@ add_task(async function test_update_browser_zoom() {
  * zoom levels when the default zoom is not at 1.0.
  */
 add_task(async function test_update_browser_zoom() {
-  await BrowserTestUtils.withNewTab(TEST_PAGE_URL, async browser => {
+  await BrowserTestUtils.withNewTab(TEST_PAGE_URL, async () => {
     let currentZoom = await FullZoomHelper.getGlobalValue();
     Assert.equal(
       currentZoom,
diff --git a/browser/base/content/test/zoom/head.js b/browser/base/content/test/zoom/head.js
index 4a42aed98f..272303de95 100644
--- a/browser/base/content/test/zoom/head.js
+++ b/browser/base/content/test/zoom/head.js
@@ -34,7 +34,7 @@ var FullZoomHelper = {
         parsedZoomValue,
         nonPrivateLoadContext,
         {
-          handleCompletion(reason) {
+          handleCompletion() {
             resolve();
           },
         }
@@ -72,7 +72,7 @@ var FullZoomHelper = {
             value = parseFloat(pref.value);
           }
         },
-        handleCompletion(reason) {
+        handleCompletion() {
           resolve(value);
         },
         handleError(error) {
@@ -84,7 +84,7 @@ var FullZoomHelper = {
 
   waitForLocationChange: function waitForLocationChange() {
     return new Promise(resolve => {
-      Services.obs.addObserver(function obs(subj, topic, data) {
+      Services.obs.addObserver(function obs(subj, topic) {
         Services.obs.removeObserver(obs, topic);
         resolve();
       }, "browser-fullZoom:location-change");
@@ -124,7 +124,7 @@ var FullZoomHelper = {
       let didLoad = false;
       let didZoom = false;
 
-      promiseTabLoadEvent(tab, url).then(event => {
+      promiseTabLoadEvent(tab, url).then(() => {
         didLoad = true;
         if (didZoom) {
           resolve();
diff --git a/browser/base/content/titlebar-items.inc.xhtml b/browser/base/content/titlebar-items.inc.xhtml
index 057fd522a9..4fea3a1266 100644
--- a/browser/base/content/titlebar-items.inc.xhtml
+++ b/browser/base/content/titlebar-items.inc.xhtml
@@ -13,7 +13,7 @@
                    data-l10n-id="browser-window-maximize-button"
                    />
     <toolbarbutton class="titlebar-button titlebar-restore"
-                   oncommand="window.fullScreen ? BrowserFullScreen() : window.restore();"
+                   oncommand="window.fullScreen ? BrowserCommands.fullScreen() : window.restore();"
                    data-l10n-id="browser-window-restore-down-button"
                    />
     <toolbarbutton class="titlebar-button titlebar-close"
diff --git a/browser/base/content/utilityOverlay.js b/browser/base/content/utilityOverlay.js
index 5967c878b3..dc3aeffaed 100644
--- a/browser/base/content/utilityOverlay.js
+++ b/browser/base/content/utilityOverlay.js
@@ -294,7 +294,7 @@ function closeMenus(node) {
  *        to check if the close command key was pressed in aEvent.
  */
 function eventMatchesKey(aEvent, aKey) {
-  let keyPressed = aKey.getAttribute("key").toLowerCase();
+  let keyPressed = (aKey.getAttribute("key") || "").toLowerCase();
   let keyModifiers = aKey.getAttribute("modifiers");
   let modifiers = ["Alt", "Control", "Meta", "Shift"];
 
@@ -341,7 +341,7 @@ function gatherTextUnder(root) {
     } else if (HTMLImageElement.isInstance(node)) {
       // If it has an "alt" attribute, add that.
       var altText = node.getAttribute("alt");
-      if (altText && altText != "") {
+      if (altText) {
         text += " " + altText;
       }
     }
diff --git a/browser/base/content/webext-panels.js b/browser/base/content/webext-panels.js
index e8820f4ad4..787193ab7d 100644
--- a/browser/base/content/webext-panels.js
+++ b/browser/base/content/webext-panels.js
@@ -125,22 +125,16 @@ function getBrowser(panel) {
   return readyPromise.then(initBrowser);
 }
 
-// Stub tabbrowser implementation for use by the tab-modal alert code.
+// Stub tabbrowser implementation to make sure that links from inside
+// extension sidebar panels open in new tabs, see bug 1488055.
 var gBrowser = {
   get selectedBrowser() {
     return document.getElementById("webext-panels-browser");
   },
 
-  getTabForBrowser(browser) {
+  getTabForBrowser() {
     return null;
   },
-
-  getTabModalPromptBox(browser) {
-    if (!browser.tabModalPromptBox) {
-      browser.tabModalPromptBox = new TabModalPromptBox(browser);
-    }
-    return browser.tabModalPromptBox;
-  },
 };
 
 function updatePosition() {
diff --git a/browser/base/content/webrtcIndicator.js b/browser/base/content/webrtcIndicator.js
index f38c7446ba..c19bfe1f35 100644
--- a/browser/base/content/webrtcIndicator.js
+++ b/browser/base/content/webrtcIndicator.js
@@ -47,7 +47,7 @@ function closingInternally() {
  * Main control object for the WebRTC global indicator
  */
 const WebRTCIndicator = {
-  init(event) {
+  init() {
     addEventListener("load", this);
     addEventListener("unload", this);
 
diff --git a/browser/base/jar.mn b/browser/base/jar.mn
index 1342630d54..9357cf1af0 100644
--- a/browser/base/jar.mn
+++ b/browser/base/jar.mn
@@ -40,6 +40,7 @@ browser.jar:
         content/browser/browser-addons.js             (content/browser-addons.js)
         content/browser/browser-allTabsMenu.js        (content/browser-allTabsMenu.js)
         content/browser/browser-captivePortal.js      (content/browser-captivePortal.js)
+        content/browser/browser-commands.js           (content/browser-commands.js)
         content/browser/browser-ctrlTab.js            (content/browser-ctrlTab.js)
         content/browser/browser-customization.js      (content/browser-customization.js)
         content/browser/browser-data-submission-info-bar.js (content/browser-data-submission-info-bar.js)
@@ -54,7 +55,6 @@ browser.jar:
         content/browser/browser-places.js                   (content/browser-places.js)
         content/browser/browser-profiles.js                 (content/browser-profiles.js)
         content/browser/browser-safebrowsing.js             (content/browser-safebrowsing.js)
-        content/browser/browser-sidebar.js                  (content/browser-sidebar.js)
         content/browser/browser-siteIdentity.js             (content/browser-siteIdentity.js)
         content/browser/browser-sitePermissionPanel.js      (content/browser-sitePermissionPanel.js)
         content/browser/browser-siteProtections.js          (content/browser-siteProtections.js)
-- 
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