add tab_count_in_alltabs_button.css

chrome/tab_count_in_alltabs_button.css

@@ -0,0 +1,25 @@
+/* Source file https://github.com/MrOtherGuy/firefox-csshacks/tree/master/chrome/tab_count_in_alltabs_button.css made available under Mozilla Public License v. 2.0
+See the above repository for updates as well as full license text. */
+
+/* Replace the icon in alltabs-button with a counter showing number of opened (visible) tabs.
+Note that you can make alltabs button always show up by setting browser.tabs.tabmanager.enabled to true */
+
+#TabsToolbar-customization-target{
+  counter-reset: tab-counter;
+}
+.tabbrowser-tab:not([hidden]){
+  counter-increment: tab-counter;
+}
+#alltabs-button >.toolbarbutton-badge-stack > .toolbarbutton-icon{
+ opacity: 0; 
+}
+#alltabs-button >.toolbarbutton-badge-stack::before{
+  content: counter(tab-counter);
+  display: flex;
+  grid-area: 1/1;
+  justify-content: center;
+  color: var(--toolbarbutton-icon-fill);
+  border-bottom: 1px solid;
+  contain: inline-size;
+}
+

html_resources/tagmap.json

@@ -168,5 +168,6 @@
 "vertical_tabs.css":["tabs","hack"],
 "bookmarks_below_content.css":["bookmarks","toolbars"],
 "overlay_fullscreen_toolbars.css":["toolbars","fullscreen"],
-"dual_color_tab_attention_indicator.css":["tab","colors"]
+"dual_color_tab_attention_indicator.css":["tab","colors"],
+"tab_count_in_alltabs_button.css":["tabs"]
 }

tags.csv

@@ -168,3 +168,4 @@ vertical_tabs.css,tabs,hack
 bookmarks_below_content.css,bookmarks,toolbars
 overlay_fullscreen_toolbars.css,toolbars,fullscreen
 dual_color_tab_attention_indicator.css,tab,colors
+tab_count_in_alltabs_button.css,tabs