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/centered_tab_label.css

@@ -3,12 +3,14 @@ See the above repository for updates as well as full license text. */
 
 .tab-label-container{
   display: grid;
-  justify-content: safe center;
-  align-items: safe center;
+  justify-content: safe center; /* Fx <112 compatibility */
+  align-items: safe center; /* Fx <112 compatibility */
+  align-content: center;
 }
 
 .tab-secondary-label{
-  -moz-box-pack: center;
+  -moz-box-pack: center; /* Fx <112 compatibility */
+  justify-content: center;
 }
 
 .tab-label,