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/hide_tabs_with_one_tab_w_window_controls.css

@@ -31,7 +31,8 @@ Window controls will be all wrong without it
 
 #TabsToolbar > .titlebar-spacer[type="post-tabs"]{
   width: calc(var(--uc-window-control-width,0px) + var(--uc-window-drag-space-post,0px)) !important;
-  -moz-box-ordinal-group: 2;
+  -moz-box-ordinal-group: 2; /* Fx < 112 compatibility */
+  order: 2;
 }
 #toolbar-menubar > spacer{ pointer-events: none }