create icon_only_tabs.css

chrome/icon_only_tabs.css

@@ -0,0 +1,14 @@
+/* Source file https://github.com/MrOtherGuy/firefox-csshacks/tree/master/chrome/icon_only_tabs.css made available under Mozilla Public License v. 2.0
+See the above repository for updates as well as full license text. */
+
+.tab-content > :not(.tab-icon-stack){
+  display: none
+}
+.tab-icon-stack > *{ margin-inline: 0 !important; }
+.tabbrowser-tab{ -moz-box-flex: 0 !important; }
+
+.tabbrowser-tab[fadein]{ min-width: calc(16px + 2 * 10px + 4px) !important; }
+:root[uidensity="compact"] .tabbrowser-tab[fadein]{
+  min-width: calc(16px + 2 * var(--inline-tab-padding,0px) + 4px) !important;
+}
+:root:not([uidensity="compact"]) .tab-content{ padding-inline: 10px !important; }

html_resources/tagmap.json

@@ -130,5 +130,6 @@
 "sharp_menupopup_corners.css":["popup","menu","proton"],
 "overlay_tab_audio_icons.css":["icon","tab"],
 "custom_default_tab_favicons.css":["tab","icon"],
-"selected_tab_as_urlbar.css":["hack"]
+"selected_tab_as_urlbar.css":["hack"],
+"icon_only_tabs.css":["tab","icon"]
 }

tags.csv

@@ -130,3 +130,4 @@ sharp_menupopup_corners.css,popup,menu,proton
 overlay_tab_audio_icons.css,icon,tab
 custom_default_tab_favicons.css,tab,icon
 selected_tab_as_urlbar.css,hack
+icon_only_tabs.css,tab,icon