Add support for modern flexbox layout

Flexbox model will replace old xul box as the default display model so
this patch adds support for that in a whole bunch of styles.

A lot of style rules are marked as "Fx < 112 compatibility" rules and
those can be removed when 112 hits release.

chrome/icon_only_tabs.css

@@ -5,10 +5,13 @@ See the above repository for updates as well as full license text. */
   display: none
 }
 .tab-icon-image:not([src]){
-  display: -moz-box !important;
+  display: block !important;
 }
 .tab-icon-stack > *{ margin-inline: 0 !important; }
-.tabbrowser-tab{ -moz-box-flex: 0 !important; }
+.tabbrowser-tab{
+  -moz-box-flex: 0 !important; /* Fx <112 compatibility */
+  flex-grow: 0 !important;
+}
 
 .tabbrowser-tab[fadein]{ min-width: calc(16px + 2 * 10px + 4px) !important; }
 :root[uidensity="compact"] .tabbrowser-tab[fadein]{