diff --git a/browser/themes/linux/browser.css b/browser/themes/linux/browser.css
index f1584d963873..09400f0cf140 100644
--- a/browser/themes/linux/browser.css
+++ b/browser/themes/linux/browser.css
@@ -2,11 +2,12 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
+@import url("chrome://browser/skin/browser-shared.css");
+
 @namespace html url("http://www.w3.org/1999/xhtml");
 
-%include ../shared/browser.inc.css
 /**
- * We intentionally do not include browser-custom-colors.inc.css,
+ * We intentionally do not include browser-custom-colors.css,
  * instead choosing to fall back to system colours and transparencies
  * in order to accomodate the wider variety of system themes on that
  * platform.
@@ -108,14 +109,7 @@ menuitem.bookmark-item {
   margin-top: -12px;
 }
 
-%include ../shared/toolbarbuttons.inc.css
-%include ../shared/toolbarbutton-icons.inc.css
-%include ../shared/menupanel.inc.css
-
 /* Address bar */
-
-%include ../shared/urlbar-searchbar.inc.css
-
 #urlbar,
 #searchbar {
   font-size: 1.05em;
@@ -131,39 +125,22 @@ menuitem.bookmark-item {
   --toolbar-field-border-color: ThreeDShadow;
 }
 
-%include ../shared/identity-block/identity-block.inc.css
-%include ../shared/notification-icons.inc.css
-%include ../shared/addon-notification.inc.css
-
 /* Address bar results view */
 
-%include ../shared/urlbarView.inc.css
-
 :root:not(:-moz-lwtheme) {
   --urlbar-popup-action-color: -moz-nativehyperlinktext;
   --autocomplete-popup-highlight-background: SelectedItem;
   --autocomplete-popup-highlight-color: SelectedItemText;
 }
 
-/* Translation infobar */
-
-%include ../shared/translation/infobar.inc.css
-
 /* AutoComplete */
 
-%include ../shared/autocomplete.inc.css
-
 #PopupAutoComplete > richlistbox > richlistitem[originaltype~="datalist-first"] {
   border-top: 1px solid ThreeDShadow;
 }
 
-/* Bookmarking panel */
-
-%include ../shared/places/editBookmarkPanel.inc.css
-
 /* Content area */
 
-%include ../shared/sidebar.inc.css
 
 #browser {
   --sidebar-border-color: ThreeDShadow;
@@ -179,8 +156,6 @@ menuitem.bookmark-item {
 
 /* Tabstrip */
 
-%include ../shared/tabs.inc.css
-
 #tabbrowser-tabs {
   /* override the global style to allow the selected tab to be above the nav-bar */
   z-index: auto;
@@ -255,16 +230,6 @@ menuitem.bookmark-item {
   margin-left: 1em;
 }
 
-%include ../shared/fullscreen/warning.inc.css
-%include ../shared/ctrlTab.inc.css
-
-/* Customization mode */
-
-%include ../shared/customizableui/customizeMode.inc.css
-
-/* End customization mode */
-
-%include ../shared/UITour.inc.css
 
 #UITourHighlight {
   /* Below are some fixes for people without an X compositor on Linux.
@@ -295,8 +260,6 @@ menuitem.bookmark-item {
   margin-bottom: -10px;
 }
 
-%include ../shared/contextmenu.inc.css
-
 #context-navigation > .menuitem-iconic > .menu-iconic-left {
   /* override toolkit/themes/linux/global/menu.css */
   padding-inline-end: 0 !important;
diff --git a/browser/themes/linux/customizableui/panelUI.css b/browser/themes/linux/customizableui/panelUI.css
index 09d3da1d6796..8055a73fdd3c 100644
--- a/browser/themes/linux/customizableui/panelUI.css
+++ b/browser/themes/linux/customizableui/panelUI.css
@@ -2,7 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-@import "chrome://browser/skin/customizableui/panelUI.inc.css";
+@import "chrome://browser/skin/customizableui/panelUI-shared.css";
 
 #customizationui-widget-panel[viewId="PanelUI-profiler"][type="arrow"][side="top"],
 #customizationui-widget-panel[viewId="PanelUI-profiler"][type="arrow"][side="bottom"] {
diff --git a/browser/themes/linux/jar.mn b/browser/themes/linux/jar.mn
index ffb5e20b8d34..4c05d37310f8 100644
--- a/browser/themes/linux/jar.mn
+++ b/browser/themes/linux/jar.mn
@@ -6,7 +6,7 @@ browser.jar:
 % skin browser classic/1.0 %skin/classic/browser/
 #include ../shared/jar.inc.mn
   skin/classic/browser/sanitizeDialog.css
-* skin/classic/browser/browser.css
+  skin/classic/browser/browser.css
   skin/classic/browser/monitor-base.png
   skin/classic/browser/monitor-border.png
   skin/classic/browser/pageInfo.css
diff --git a/browser/themes/osx/browser.css b/browser/themes/osx/browser.css
index 4be677928d1f..e39e14ca04ed 100644
--- a/browser/themes/osx/browser.css
+++ b/browser/themes/osx/browser.css
@@ -2,10 +2,10 @@
  * 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/. */
 
-@namespace html url("http://www.w3.org/1999/xhtml");
+@import url("chrome://browser/skin/browser-shared.css");
+@import url("chrome://browser/skin/browser-custom-colors.css");
 
-%include ../shared/browser.inc.css
-%include ../shared/browser-custom-colors.inc.css
+@namespace html url("http://www.w3.org/1999/xhtml");
 
 :root {
   --toolbar-non-lwt-bgcolor: -moz-dialog;
@@ -186,10 +186,6 @@
   margin-top: -8px;
 }
 
-%include ../shared/toolbarbuttons.inc.css
-%include ../shared/toolbarbutton-icons.inc.css
-%include ../shared/menupanel.inc.css
-
 /* Inactive elements are faded out on OSX */
 .toolbarbutton-1:not(:hover):-moz-window-inactive,
 .bookmark-item:not(:hover):-moz-window-inactive,
@@ -199,8 +195,6 @@
 
 /* Address bar */
 
-%include ../shared/urlbar-searchbar.inc.css
-
 @media not (prefers-contrast) {
   :root:not(:-moz-lwtheme) #urlbar {
     --urlbar-box-bgcolor: white;
@@ -246,18 +240,8 @@ moz-input-box > menupopup .context-menu-add-engine > .menu-iconic-left {
   margin-inline: 0 5px;
 }
 
-%include ../shared/identity-block/identity-block.inc.css
-%include ../shared/notification-icons.inc.css
-%include ../shared/addon-notification.inc.css
-
-/* Address bar results view */
-
-%include ../shared/urlbarView.inc.css
-
 /* ----- AUTOCOMPLETE ----- */
 
-%include ../shared/autocomplete.inc.css
-
 #PopupAutoComplete > richlistbox > richlistitem[originaltype~="datalist-first"] {
   border-top: 1px solid #C7C7C7;
 }
@@ -274,7 +258,6 @@ moz-input-box > menupopup .context-menu-add-engine > .menu-iconic-left {
 
 /* Bookmarking panel */
 
-%include ../shared/places/editBookmarkPanel.inc.css
 
 /* The following elements come from editBookmarkPanel.inc.xhtml. Styling that's
    specific to the editBookmarkPanel should be in browser.css. Styling that
@@ -356,8 +339,6 @@ moz-input-box > menupopup .context-menu-add-engine > .menu-iconic-left {
 
 /* ----- SIDEBAR ELEMENTS ----- */
 
-%include ../shared/sidebar.inc.css
-
 #browser {
   --sidebar-border-color: hsla(240, 5%, 5%, .1);
 }
@@ -390,8 +371,6 @@ moz-input-box > menupopup .context-menu-add-engine > .menu-iconic-left {
   margin: 0;
 }
 
-%include ../shared/tabs.inc.css
-
 .tab-label {
   margin-block: 1px 0;
 }
@@ -442,10 +421,6 @@ moz-input-box > menupopup .context-menu-add-engine > .menu-iconic-left {
   color: SelectedItemText !important;
 }
 
-/* Translation */
-
-%include ../shared/translation/infobar.inc.css
-
 /* Status panel */
 
 #statuspanel-label {
@@ -480,9 +455,6 @@ moz-input-box > menupopup .context-menu-add-engine > .menu-iconic-left {
   margin-left: 1em;
 }
 
-%include ../shared/fullscreen/warning.inc.css
-%include ../shared/ctrlTab.inc.css
-
 /* On mac, the popup notification contents are indented by default and so
   the default closebutton margins from notification.css require adjustment */
 
@@ -491,14 +463,6 @@ moz-input-box > menupopup .context-menu-add-engine > .menu-iconic-left {
   margin-top: -7px;
 }
 
-/* Customization mode */
-
-%include ../shared/customizableui/customizeMode.inc.css
-
-/* End customization mode */
-
-%include ../shared/UITour.inc.css
-
 #UITourTooltipDescription {
   font-size: 1.18rem;
   line-height: 2rem;
diff --git a/browser/themes/osx/customizableui/panelUI.css b/browser/themes/osx/customizableui/panelUI.css
index f4ec8cde2c14..65840b830653 100644
--- a/browser/themes/osx/customizableui/panelUI.css
+++ b/browser/themes/osx/customizableui/panelUI.css
@@ -2,7 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-@import "chrome://browser/skin/customizableui/panelUI.inc.css";
+@import "chrome://browser/skin/customizableui/panelUI-shared.css";
 
 #customizationui-widget-panel[viewId="PanelUI-profiler"][type="arrow"][side="top"],
 #customizationui-widget-panel[viewId="PanelUI-profiler"][type="arrow"][side="bottom"] {
@@ -22,11 +22,11 @@
 #permission-popup,
 #protections-popup,
 #appMenu-notification-popup {
-  margin-top: -1px; /* Overrides value from panelUI.inc.css */
+  margin-top: -1px; /* Overrides value from panelUI-shared.css */
 }
 
 #BMB_bookmarksPopup {
-  margin-top: -4px; /* Overrides value from panelUI.inc.css */
+  margin-top: -4px; /* Overrides value from panelUI-shared.css */
 }
 
 .panel-subview-body {
diff --git a/browser/themes/osx/jar.mn b/browser/themes/osx/jar.mn
index 701e3f4ee455..6245c67a8d29 100644
--- a/browser/themes/osx/jar.mn
+++ b/browser/themes/osx/jar.mn
@@ -6,7 +6,8 @@ browser.jar:
 % skin browser classic/1.0 %skin/classic/browser/
 #include ../shared/jar.inc.mn
   skin/classic/browser/sanitizeDialog.css
-* skin/classic/browser/browser.css
+  skin/classic/browser/browser.css
+  skin/classic/browser/browser-custom-colors.css     (../shared/browser-custom-colors.css)
   skin/classic/browser/pageInfo.css
   skin/classic/browser/customizableui/panelUI.css    (customizableui/panelUI.css)
   skin/classic/browser/downloads/allDownloadsView.css (downloads/allDownloadsView.css)
diff --git a/browser/themes/shared/UITour.inc.css b/browser/themes/shared/UITour.css
similarity index 98%
rename from browser/themes/shared/UITour.inc.css
rename to browser/themes/shared/UITour.css
index 8ea3835982b5..3380e7d4a0bb 100644
--- a/browser/themes/shared/UITour.inc.css
+++ b/browser/themes/shared/UITour.css
@@ -2,6 +2,7 @@
  * 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/. */
 
+@namespace html url("http://www.w3.org/1999/xhtml");
 
 /* UI Tour */
 
diff --git a/browser/themes/shared/addon-notification.inc.css b/browser/themes/shared/addon-notification.css
similarity index 95%
rename from browser/themes/shared/addon-notification.inc.css
rename to browser/themes/shared/addon-notification.css
index 91536c34d0d5..79c560b513e0 100644
--- a/browser/themes/shared/addon-notification.inc.css
+++ b/browser/themes/shared/addon-notification.css
@@ -2,6 +2,8 @@
  * 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/. */
 
+@namespace html url("http://www.w3.org/1999/xhtml");
+
 .popup-notification-body[popupid="addon-progress"],
 .popup-notification-body[popupid="addon-install-confirmation"] {
   width: 28em;
diff --git a/browser/themes/shared/addons/extension-controlled.inc.css b/browser/themes/shared/addons/extension-controlled.css
similarity index 100%
rename from browser/themes/shared/addons/extension-controlled.inc.css
rename to browser/themes/shared/addons/extension-controlled.css
diff --git a/browser/themes/shared/autocomplete.inc.css b/browser/themes/shared/autocomplete.css
similarity index 99%
rename from browser/themes/shared/autocomplete.inc.css
rename to browser/themes/shared/autocomplete.css
index 66629fe8312c..4cd394a1d250 100644
--- a/browser/themes/shared/autocomplete.inc.css
+++ b/browser/themes/shared/autocomplete.css
@@ -2,6 +2,8 @@
  * 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/. */
 
+@namespace html url("http://www.w3.org/1999/xhtml");
+
 /* General popup rules */
 
 #PopupAutoComplete > richlistbox > richlistitem {
diff --git a/browser/themes/shared/browser-custom-colors.inc.css b/browser/themes/shared/browser-custom-colors.css
similarity index 98%
rename from browser/themes/shared/browser-custom-colors.inc.css
rename to browser/themes/shared/browser-custom-colors.css
index 7793730874a3..91f3d8dfcd9b 100644
--- a/browser/themes/shared/browser-custom-colors.inc.css
+++ b/browser/themes/shared/browser-custom-colors.css
@@ -2,6 +2,8 @@
  * 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/. */
 
+@namespace html url("http://www.w3.org/1999/xhtml");
+
 @media not (prefers-contrast) {
   :root:not(:-moz-lwtheme) {
     --button-primary-bgcolor: rgb(0,97,224);
diff --git a/browser/themes/shared/browser.inc.css b/browser/themes/shared/browser-shared.css
similarity index 94%
rename from browser/themes/shared/browser.inc.css
rename to browser/themes/shared/browser-shared.css
index cf5cdb9467bb..176c381f4da6 100644
--- a/browser/themes/shared/browser.inc.css
+++ b/browser/themes/shared/browser-shared.css
@@ -2,10 +2,30 @@
  * 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/. */
 
-%include downloads/indicator.inc.css
-%include addons/extension-controlled.inc.css
+@import url("chrome://browser/skin/downloads/indicator.css");
+@import url("chrome://browser/skin/addons/extension-controlled.css");
+@import url("chrome://browser/skin/places/places-tooltip.css");
+@import url("chrome://browser/skin/toolbarbuttons.css");
+@import url("chrome://browser/skin/toolbarbutton-icons.css");
+@import url("chrome://browser/skin/menupanel.css");
+@import url("chrome://browser/skin/urlbar-searchbar.css");
+@import url("chrome://browser/skin/identity-block/identity-block.css");
+@import url("chrome://browser/skin/notification-icons.css");
+@import url("chrome://browser/skin/addon-notification.css");
+@import url("chrome://browser/skin/urlbarView.css");
+@import url("chrome://browser/skin/translation/infobar.css");
+@import url("chrome://browser/skin/autocomplete.css");
+@import url("chrome://browser/skin/places/editBookmarkPanel.css");
+@import url("chrome://browser/skin/sidebar.css");
+@import url("chrome://browser/skin/tabs.css");
+@import url("chrome://browser/skin/fullscreen/warning.css");
+@import url("chrome://browser/skin/ctrlTab.css");
+@import url("chrome://browser/skin/customizableui/customizeMode.css");
+@import url("chrome://browser/skin/UITour.css");
+@import url("chrome://browser/skin/contextmenu.css");
+
+@namespace html url("http://www.w3.org/1999/xhtml");
 
-%filter substitution
 :root {
   --toolbar-bgcolor: var(--toolbar-non-lwt-bgcolor);
   --toolbar-bgimage: var(--toolbar-non-lwt-bgimage);
@@ -170,10 +190,6 @@ panel .text-link:-moz-lwtheme:not(:hover) {
   transition: var(--ext-theme-background-transition);
 }
 
-/* Places tooltips */
-
-%include places/places-tooltip.inc.css
-
 /* Bookmark toolbar */
 
 #PersonalToolbar {
diff --git a/browser/themes/shared/contextmenu.inc.css b/browser/themes/shared/contextmenu.css
similarity index 96%
rename from browser/themes/shared/contextmenu.inc.css
rename to browser/themes/shared/contextmenu.css
index 9981b8353289..b0ace28d12b8 100644
--- a/browser/themes/shared/contextmenu.inc.css
+++ b/browser/themes/shared/contextmenu.css
@@ -2,6 +2,8 @@
  * 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/. */
 
+@namespace html url("http://www.w3.org/1999/xhtml");
+
 #context-navigation > .menuitem-iconic {
   -moz-box-flex: 1;
   -moz-box-pack: center;
diff --git a/browser/themes/shared/ctrlTab.inc.css b/browser/themes/shared/ctrlTab.css
similarity index 97%
rename from browser/themes/shared/ctrlTab.inc.css
rename to browser/themes/shared/ctrlTab.css
index 48e842cb993c..fa3b86221a64 100644
--- a/browser/themes/shared/ctrlTab.inc.css
+++ b/browser/themes/shared/ctrlTab.css
@@ -2,6 +2,8 @@
  * 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/. */
 
+@namespace html url("http://www.w3.org/1999/xhtml");
+
 /* Ctrl-Tab */
 
 #ctrlTab-panel {
diff --git a/browser/themes/shared/customizableui/customizeMode.inc.css b/browser/themes/shared/customizableui/customizeMode.css
similarity index 99%
rename from browser/themes/shared/customizableui/customizeMode.inc.css
rename to browser/themes/shared/customizableui/customizeMode.css
index 0b3839ed17be..60866985cf0b 100644
--- a/browser/themes/shared/customizableui/customizeMode.inc.css
+++ b/browser/themes/shared/customizableui/customizeMode.css
@@ -2,6 +2,8 @@
  * 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/. */
 
+@namespace html url("http://www.w3.org/1999/xhtml");
+
 /* Customization mode */
 
 :root {
diff --git a/browser/themes/shared/customizableui/panelUI.inc.css b/browser/themes/shared/customizableui/panelUI-shared.css
similarity index 100%
rename from browser/themes/shared/customizableui/panelUI.inc.css
rename to browser/themes/shared/customizableui/panelUI-shared.css
diff --git a/browser/themes/shared/downloads/indicator.inc.css b/browser/themes/shared/downloads/indicator.css
similarity index 100%
rename from browser/themes/shared/downloads/indicator.inc.css
rename to browser/themes/shared/downloads/indicator.css
diff --git a/browser/themes/shared/fullscreen/warning.inc.css b/browser/themes/shared/fullscreen/warning.css
similarity index 96%
rename from browser/themes/shared/fullscreen/warning.inc.css
rename to browser/themes/shared/fullscreen/warning.css
index 5e04b4347e2f..fc1c203a7888 100644
--- a/browser/themes/shared/fullscreen/warning.inc.css
+++ b/browser/themes/shared/fullscreen/warning.css
@@ -2,6 +2,7 @@
  * 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/. */
 
+@namespace html url("http://www.w3.org/1999/xhtml");
 
 html|*.pointerlockfswarning {
   align-items: center;
diff --git a/browser/themes/shared/identity-block/identity-block.inc.css b/browser/themes/shared/identity-block/identity-block.css
similarity index 99%
rename from browser/themes/shared/identity-block/identity-block.inc.css
rename to browser/themes/shared/identity-block/identity-block.css
index ee572ad7aa0b..c30d70b8f935 100644
--- a/browser/themes/shared/identity-block/identity-block.inc.css
+++ b/browser/themes/shared/identity-block/identity-block.css
@@ -2,8 +2,7 @@
  * 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/. */
 
-
-%filter substitution
+@namespace html url("http://www.w3.org/1999/xhtml");
 
 #identity-box {
   margin-inline-end: var(--identity-box-margin-inline);
diff --git a/browser/themes/shared/jar.inc.mn b/browser/themes/shared/jar.inc.mn
index cd257d6fb439..178c01cc4c25 100644
--- a/browser/themes/shared/jar.inc.mn
+++ b/browser/themes/shared/jar.inc.mn
@@ -13,18 +13,34 @@
   skin/classic/browser/aboutSessionRestore.css                 (../shared/aboutSessionRestore.css)
   skin/classic/browser/aboutTabCrashed.css                     (../shared/aboutTabCrashed.css)
   skin/classic/browser/aboutWelcomeBack.css                    (../shared/aboutWelcomeBack.css)
+  skin/classic/browser/addon-notification.css                  (../shared/addon-notification.css)
+  skin/classic/browser/autocomplete.css                        (../shared/autocomplete.css)
   skin/classic/browser/blockedSite.css                         (../shared/blockedSite.css)
+  skin/classic/browser/browser-shared.css                      (../shared/browser-shared.css)
+  skin/classic/browser/contextmenu.css                         (../shared/contextmenu.css)
+  skin/classic/browser/ctrlTab.css                             (../shared/ctrlTab.css)
   skin/classic/browser/light-dark-overrides.css          (../shared/light-dark-overrides.css)
   skin/classic/browser/error-pages.css                         (../shared/error-pages.css)
+  skin/classic/browser/menupanel.css                           (../shared/menupanel.css)
+  skin/classic/browser/notification-icons.css                  (../shared/notification-icons.css)
   skin/classic/browser/offlineSupportPages.css                 (../shared/offlineSupportPages.css)
   skin/classic/browser/searchbar.css                           (../shared/searchbar.css)
   skin/classic/browser/setDesktopBackground.css                (../shared/setDesktopBackground.css)
+  skin/classic/browser/sidebar.css                             (../shared/sidebar.css)
+  skin/classic/browser/tabs.css                                (../shared/tabs.css)
+  skin/classic/browser/toolbarbuttons.css                      (../shared/toolbarbuttons.css)
+  skin/classic/browser/toolbarbutton-icons.css                 (../shared/toolbarbutton-icons.css)
+  skin/classic/browser/urlbar-dynamic-results.css              (../shared/urlbar-dynamic-results.css)
+  skin/classic/browser/urlbar-searchbar.css                    (../shared/urlbar-searchbar.css)
+  skin/classic/browser/urlbarView.css                          (../shared/urlbarView.css)
+  skin/classic/browser/UITour.css                              (../shared/UITour.css)
   skin/classic/browser/webRTC-indicator.css                    (../shared/webRTC-indicator.css)
   skin/classic/browser/addons/addon-install-blocked.svg        (../shared/addons/addon-install-blocked.svg)
   skin/classic/browser/addons/addon-install-downloading.svg    (../shared/addons/addon-install-downloading.svg)
   skin/classic/browser/addons/addon-install-error.svg          (../shared/addons/addon-install-error.svg)
   skin/classic/browser/addons/addon-install-installed.svg      (../shared/addons/addon-install-installed.svg)
   skin/classic/browser/addons/addon-install-warning.svg        (../shared/addons/addon-install-warning.svg)
+  skin/classic/browser/addons/extension-controlled.css         (../shared/addons/extension-controlled.css)
   skin/classic/browser/controlcenter/3rdpartycookies.svg       (../shared/controlcenter/3rdpartycookies.svg)
   skin/classic/browser/controlcenter/cryptominers.svg          (../shared/controlcenter/cryptominers.svg)
   skin/classic/browser/controlcenter/dashboard.svg             (../shared/controlcenter/dashboard.svg)
@@ -33,7 +49,9 @@
   skin/classic/browser/controlcenter/panel.css                 (../shared/controlcenter/panel.css)
   skin/classic/browser/controlcenter/tracking-protection.svg   (../shared/controlcenter/tracking-protection.svg)
   skin/classic/browser/controlcenter/hero-message-background.svg   (../shared/controlcenter/hero-message-background.svg)
-  skin/classic/browser/customizableui/panelUI.inc.css          (../shared/customizableui/panelUI.inc.css)
+  skin/classic/browser/customizableui/customizeMode.css        (../shared/customizableui/customizeMode.css)
+  skin/classic/browser/customizableui/panelUI-shared.css       (../shared/customizableui/panelUI-shared.css)
+
   skin/classic/browser/customizableui/empty-overflow-panel.png     (../shared/customizableui/empty-overflow-panel.png)
   skin/classic/browser/customizableui/empty-overflow-panel@2x.png  (../shared/customizableui/empty-overflow-panel@2x.png)
   skin/classic/browser/customizableui/density-compact.svg      (../shared/customizableui/density-compact.svg)
@@ -46,10 +64,12 @@
   skin/classic/browser/downloads/download-summary.svg          (../shared/downloads/download-summary.svg)
   skin/classic/browser/downloads/downloads.inc.css             (../shared/downloads/downloads.inc.css)
   skin/classic/browser/downloads/downloads.svg                 (../shared/downloads/downloads.svg)
+  skin/classic/browser/downloads/indicator.css                 (../shared/downloads/indicator.css)
   skin/classic/browser/downloads/notification-finish-animation.svg       (../shared/downloads/notification-finish-animation.svg)
   skin/classic/browser/downloads/notification-start-animation.svg        (../shared/downloads/notification-start-animation.svg)
   skin/classic/browser/downloads/progressmeter.css             (../shared/downloads/progressmeter.css)
   skin/classic/browser/drm-icon.svg                            (../shared/drm-icon.svg)
+  skin/classic/browser/identity-block/identity-block.css       (../shared/identity-block/identity-block.css)
   skin/classic/browser/permissions.svg                         (../shared/identity-block/permissions.svg)
   skin/classic/browser/notification-icons/autoplay-media.svg                (../shared/notification-icons/autoplay-media.svg)
   skin/classic/browser/notification-icons/autoplay-media-blocked.svg        (../shared/notification-icons/autoplay-media-blocked.svg)
@@ -77,10 +97,12 @@
   skin/classic/browser/tracking-protection-disabled.svg               (../shared/identity-block/tracking-protection-disabled.svg)
   skin/classic/browser/tracking-protection-active.svg                 (../shared/identity-block/tracking-protection-active.svg)
   skin/classic/browser/tracking-protection-active-brighttext.svg      (../shared/identity-block/tracking-protection-active-brighttext.svg)
+  skin/classic/browser/translation/infobar.css                        (../shared/translation/infobar.css)
+
   skin/classic/browser/toolbar-drag-indicator.svg                     (../shared/toolbar-drag-indicator.svg)
 
   skin/classic/browser/protections/resolved-breach.svg         (../shared/protections/resolved-breach.svg)
-  skin/classic/browser/protections/resolved-breach-gray.svg         (../shared/protections/resolved-breach-gray.svg)
+  skin/classic/browser/protections/resolved-breach-gray.svg    (../shared/protections/resolved-breach-gray.svg)
   skin/classic/browser/protections/breached-password.svg       (../shared/protections/breached-password.svg)
   skin/classic/browser/protections/new-feature.svg             (../shared/protections/new-feature.svg)
 
@@ -131,6 +153,8 @@
   skin/classic/browser/fxa/monitor.svg                         (../shared/fxa/monitor.svg)
   skin/classic/browser/fxa/add-device.svg                      (../shared/fxa/add-device.svg)
 
+  skin/classic/browser/fullscreen/warning.css                  (../shared/fullscreen/warning.css)
+
   skin/classic/browser/add-circle-fill.svg            (../shared/icons/add-circle-fill.svg)
   skin/classic/browser/back.svg                       (../shared/icons/back.svg)
   skin/classic/browser/bookmark.svg                   (../shared/icons/bookmark.svg)
@@ -229,7 +253,9 @@
   skin/classic/browser/places/sidebar.css                     (../shared/places/sidebar.css)
   skin/classic/browser/places/bookmarksMenu.svg                (../shared/places/bookmarksMenu.svg)
   skin/classic/browser/places/bookmarksToolbar.svg             (../shared/places/bookmarksToolbar.svg)
+  skin/classic/browser/places/editBookmarkPanel.css            (../shared/places/editBookmarkPanel.css)
   skin/classic/browser/places/folder-smart.svg                 (../shared/places/folder-smart.svg)
+  skin/classic/browser/places/places-tooltip.css               (../shared/places/places-tooltip.css)
   skin/classic/browser/places/tag.svg                          (../shared/places/tag.svg)
   skin/classic/browser/places/tree-icons.css                   (../shared/places/tree-icons.css)
   skin/classic/browser/privatebrowsing/aboutPrivateBrowsing.css (../shared/privatebrowsing/aboutPrivateBrowsing.css)
diff --git a/browser/themes/shared/menupanel.inc.css b/browser/themes/shared/menupanel.css
similarity index 93%
rename from browser/themes/shared/menupanel.inc.css
rename to browser/themes/shared/menupanel.css
index 4629e5e86891..e20b3cfaa433 100644
--- a/browser/themes/shared/menupanel.inc.css
+++ b/browser/themes/shared/menupanel.css
@@ -2,6 +2,8 @@
  * 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/. */
 
+@namespace html url("http://www.w3.org/1999/xhtml");
+
 /* Menu panel and palette styles */
 
 /**
diff --git a/browser/themes/shared/notification-icons.inc.css b/browser/themes/shared/notification-icons.css
similarity index 99%
rename from browser/themes/shared/notification-icons.inc.css
rename to browser/themes/shared/notification-icons.css
index f401ac0ca1bf..04e01dcc1c5a 100644
--- a/browser/themes/shared/notification-icons.inc.css
+++ b/browser/themes/shared/notification-icons.css
@@ -2,6 +2,8 @@
  * 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/. */
 
+@namespace html url("http://www.w3.org/1999/xhtml");
+
 #notification-popup-box {
   height: calc(var(--urlbar-min-height) - 2px /* border */ - 2 * var(--urlbar-container-padding));
   -moz-context-properties: fill, fill-opacity;
diff --git a/browser/themes/shared/places/editBookmarkPanel.inc.css b/browser/themes/shared/places/editBookmarkPanel.css
similarity index 99%
rename from browser/themes/shared/places/editBookmarkPanel.inc.css
rename to browser/themes/shared/places/editBookmarkPanel.css
index 147f54197d3c..365f1b216560 100644
--- a/browser/themes/shared/places/editBookmarkPanel.inc.css
+++ b/browser/themes/shared/places/editBookmarkPanel.css
@@ -2,6 +2,7 @@
  * 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/. */
 
+@namespace html url("http://www.w3.org/1999/xhtml");
 
 /**
  * Our OS integration code toggles whether system colors should return light or
diff --git a/browser/themes/shared/places/places-tooltip.inc.css b/browser/themes/shared/places/places-tooltip.css
similarity index 100%
rename from browser/themes/shared/places/places-tooltip.inc.css
rename to browser/themes/shared/places/places-tooltip.css
diff --git a/browser/themes/shared/sidebar.inc.css b/browser/themes/shared/sidebar.css
similarity index 98%
rename from browser/themes/shared/sidebar.inc.css
rename to browser/themes/shared/sidebar.css
index 47e43da81708..5a41894965d7 100644
--- a/browser/themes/shared/sidebar.inc.css
+++ b/browser/themes/shared/sidebar.css
@@ -2,6 +2,8 @@
  * 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/. */
 
+@namespace html url("http://www.w3.org/1999/xhtml");
+
 #sidebar-box {
   --sidebar-background-color: Field;
   --sidebar-text-color: FieldText;
diff --git a/browser/themes/shared/tabs.inc.css b/browser/themes/shared/tabs.css
similarity index 99%
rename from browser/themes/shared/tabs.inc.css
rename to browser/themes/shared/tabs.css
index d3b4d3fe8f79..be7b5e2b2e89 100644
--- a/browser/themes/shared/tabs.inc.css
+++ b/browser/themes/shared/tabs.css
@@ -2,6 +2,8 @@
  * 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/. */
 
+@namespace html url("http://www.w3.org/1999/xhtml");
+
 :root {
   --tab-min-height: 36px;
   --inline-tab-padding: 8px;
diff --git a/browser/themes/shared/toolbarbutton-icons.inc.css b/browser/themes/shared/toolbarbutton-icons.css
similarity index 99%
rename from browser/themes/shared/toolbarbutton-icons.inc.css
rename to browser/themes/shared/toolbarbutton-icons.css
index afbaa167257f..2eb77b9168f6 100644
--- a/browser/themes/shared/toolbarbutton-icons.inc.css
+++ b/browser/themes/shared/toolbarbutton-icons.css
@@ -2,6 +2,8 @@
  * 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/. */
 
+@namespace html url("http://www.w3.org/1999/xhtml");
+
 :root {
   --toolbarbutton-icon-fill-attention: -moz-accent-color;
   --warning-icon-bgcolor: #FFA436;
diff --git a/browser/themes/shared/toolbarbuttons.inc.css b/browser/themes/shared/toolbarbuttons.css
similarity index 99%
rename from browser/themes/shared/toolbarbuttons.inc.css
rename to browser/themes/shared/toolbarbuttons.css
index 0fcd141eee44..6ef769658fb9 100644
--- a/browser/themes/shared/toolbarbuttons.inc.css
+++ b/browser/themes/shared/toolbarbuttons.css
@@ -2,6 +2,8 @@
  * 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/. */
 
+@namespace html url("http://www.w3.org/1999/xhtml");
+
 :root {
   --toolbar-start-end-padding: 8px;
   --toolbarbutton-outer-padding: 2px;
diff --git a/browser/themes/shared/translation/infobar.inc.css b/browser/themes/shared/translation/infobar.css
similarity index 97%
rename from browser/themes/shared/translation/infobar.inc.css
rename to browser/themes/shared/translation/infobar.css
index f43804d2829a..7f42cf29391a 100644
--- a/browser/themes/shared/translation/infobar.inc.css
+++ b/browser/themes/shared/translation/infobar.css
@@ -2,6 +2,8 @@
  * 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/. */
 
+@namespace html url("http://www.w3.org/1999/xhtml");
+
 notification[value="translation"] .messageImage {
   list-style-image: url(chrome://browser/skin/translation-16.png) !important;
   -moz-image-region: rect(0, 32px, 16px, 16px);
diff --git a/browser/themes/shared/urlbar/dynamicResults.inc.css b/browser/themes/shared/urlbar-dynamic-results.css
similarity index 100%
rename from browser/themes/shared/urlbar/dynamicResults.inc.css
rename to browser/themes/shared/urlbar-dynamic-results.css
diff --git a/browser/themes/shared/urlbar-searchbar.inc.css b/browser/themes/shared/urlbar-searchbar.css
similarity index 99%
rename from browser/themes/shared/urlbar-searchbar.inc.css
rename to browser/themes/shared/urlbar-searchbar.css
index c71aa4870273..5088ebdd53f7 100644
--- a/browser/themes/shared/urlbar-searchbar.inc.css
+++ b/browser/themes/shared/urlbar-searchbar.css
@@ -2,6 +2,8 @@
  * 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/. */
 
+@namespace html url("http://www.w3.org/1999/xhtml");
+
 :root {
   --toolbar-field-border-color: hsla(240,5%,5%,.25);
   --toolbar-field-focus-border-color: var(--focus-outline-color);
diff --git a/browser/themes/shared/urlbarView.inc.css b/browser/themes/shared/urlbarView.css
similarity index 99%
rename from browser/themes/shared/urlbarView.inc.css
rename to browser/themes/shared/urlbarView.css
index 1607813df447..85fb5cb12e76 100644
--- a/browser/themes/shared/urlbarView.inc.css
+++ b/browser/themes/shared/urlbarView.css
@@ -2,7 +2,9 @@
  * 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/. */
 
-%include ../shared/urlbar/dynamicResults.inc.css
+@import url("chrome://browser/skin/urlbar-dynamic-results.css");
+
+@namespace html url("http://www.w3.org/1999/xhtml");
 
 :root {
   --autocomplete-popup-highlight-background: SelectedItem;
diff --git a/browser/themes/windows/browser.css b/browser/themes/windows/browser.css
index db6a09f2ef81..18d1a8270674 100644
--- a/browser/themes/windows/browser.css
+++ b/browser/themes/windows/browser.css
@@ -2,11 +2,11 @@
  * 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/. */
 
-@namespace html url("http://www.w3.org/1999/xhtml");
+@import url("chrome://browser/skin/browser-shared.css");
+@import url("chrome://browser/skin/browser-custom-colors.css");
+@import url("chrome://browser/skin/browser-aero.css");
 
-%include ../shared/browser.inc.css
-%include ../shared/browser-custom-colors.inc.css
-%filter substitution
+@namespace html url("http://www.w3.org/1999/xhtml");
 
 :root {
   --toolbar-non-lwt-bgcolor: -moz-dialog;
@@ -303,14 +303,8 @@ menuitem.bookmark-item {
 }
 
 
-%include ../shared/toolbarbuttons.inc.css
-%include ../shared/toolbarbutton-icons.inc.css
-%include ../shared/menupanel.inc.css
-
 /* Address bar */
 
-%include ../shared/urlbar-searchbar.inc.css
-
 @media not (prefers-contrast) {
   :root:not(:-moz-lwtheme) #urlbar {
     --urlbar-box-bgcolor: white;
@@ -347,14 +341,8 @@ menuitem.bookmark-item {
   }
 }
 
-%include ../shared/identity-block/identity-block.inc.css
-%include ../shared/notification-icons.inc.css
-%include ../shared/addon-notification.inc.css
-
 /* Address bar results view */
 
-%include ../shared/urlbarView.inc.css
-
 @media (-moz-windows-default-theme: 0) {
   :root:not(:-moz-lwtheme) {
     --urlbar-popup-action-color: -moz-nativehyperlinktext;
@@ -363,20 +351,12 @@ menuitem.bookmark-item {
 
 /* Autocomplete */
 
-%include ../shared/autocomplete.inc.css
-
 #PopupAutoComplete > richlistbox > richlistitem[originaltype~="datalist-first"] {
   border-top: 1px solid ThreeDShadow;
 }
 
-/* Bookmarking panel */
-
-%include ../shared/places/editBookmarkPanel.inc.css
-
 /* Content area */
 
-%include ../shared/sidebar.inc.css
-
 #browser {
   --sidebar-border-color: ThreeDLightShadow;
 }
@@ -388,8 +368,6 @@ menuitem.bookmark-item {
   padding: 0;
 }
 
-%include ../shared/tabs.inc.css
-
 @media (-moz-windows-default-theme: 0) {
   /* For high contrast themes. */
   #tabbrowser-tabpanels,
@@ -411,10 +389,6 @@ toolbarbutton.bookmark-item[dragover="true"][open="true"] {
   fill: SelectedItemText;
 }
 
-/* Translation infobar */
-
-%include ../shared/translation/infobar.inc.css
-
 /* Bookmarks roots menu-items */
 
 #bookmarksToolbarFolderMenu,
@@ -473,15 +447,6 @@ toolbarbutton.bookmark-item[dragover="true"][open="true"] {
   margin-left: 1em;
 }
 
-%include ../shared/fullscreen/warning.inc.css
-%include ../shared/ctrlTab.inc.css
-
-/* Customization mode */
-
-%include ../shared/customizableui/customizeMode.inc.css
-
-/* End customization mode */
-
 /* Prevent window controls from overlapping the nav bar's shadow on the tab
  * bar. */
 #TabsToolbar > .titlebar-buttonbox-container {
@@ -500,8 +465,6 @@ toolbarbutton.bookmark-item[dragover="true"][open="true"] {
 
 /* End private browsing and accessibility indicators */
 
-%include ../shared/UITour.inc.css
-
 #UITourTooltipButtons {
   /**
    * Override the --arrowpanel-padding so the background extends
@@ -511,8 +474,6 @@ toolbarbutton.bookmark-item[dragover="true"][open="true"] {
   margin-bottom: -10px;
 }
 
-%include ../shared/contextmenu.inc.css
-
 /* Make menu items larger when opened through touch. */
 #widget-overflow[touchmode] .toolbarbutton-1,
 panel[touchmode] .PanelUI-subView .subviewbutton,
@@ -623,8 +584,6 @@ menupopup[touchmode] menuitem {
   }
 }
 
-%include browser-aero.css
-
 .cui-widget-panelview[id^=PanelUI-webext-] {
   border-radius: var(--arrowpanel-border-radius);
 }
diff --git a/browser/themes/windows/customizableui/panelUI.css b/browser/themes/windows/customizableui/panelUI.css
index b699bbaa8c9a..1691fd8bcba0 100644
--- a/browser/themes/windows/customizableui/panelUI.css
+++ b/browser/themes/windows/customizableui/panelUI.css
@@ -2,7 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-@import "chrome://browser/skin/customizableui/panelUI.inc.css";
+@import "chrome://browser/skin/customizableui/panelUI-shared.css";
 
 #customizationui-widget-panel[viewId="PanelUI-profiler"][type="arrow"][side="top"],
 #customizationui-widget-panel[viewId="PanelUI-profiler"][type="arrow"][side="bottom"] {
diff --git a/browser/themes/windows/jar.mn b/browser/themes/windows/jar.mn
index e3461a4aa2ad..34e36ec50eb4 100644
--- a/browser/themes/windows/jar.mn
+++ b/browser/themes/windows/jar.mn
@@ -6,7 +6,9 @@ browser.jar:
 % skin browser classic/1.0 %skin/classic/browser/
 #include ../shared/jar.inc.mn
   skin/classic/browser/sanitizeDialog.css
-* skin/classic/browser/browser.css
+  skin/classic/browser/browser.css
+  skin/classic/browser/browser-custom-colors.css  (../shared/browser-custom-colors.css)
+  skin/classic/browser/browser-aero.css
   skin/classic/browser/monitor-base.png
   skin/classic/browser/monitor-border.png
   skin/classic/browser/pageInfo.css