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/selected_tab_gradient_border.css

@@ -12,7 +12,7 @@ See the above repository for updates as well as full license text. */
 .tab-background[selected] > .tab-context-line{ margin: -5px 0 0 !important; }
 .tabbrowser-tab[selected] > .tab-stack::before{
   content: "";
-  display: -moz-box;
+  display: flex;
   min-height: inherit;
   border-radius: var(--tab-border-radius);
   grid-area: 1/1;