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/tab_close_button_always_on_hover.css

@@ -6,5 +6,6 @@ See the above repository for updates as well as full license text. */
   display:none;
 }
 .tabbrowser-tab:not([pinned]):hover .tab-close-button{
-  display:-moz-box !important;
+  display: flex !important;
+  align-items: center;
 }