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.css

@@ -42,7 +42,8 @@ This will NOT work correctly if you have hidden tabs such as with tab groups ext
 }
 @media (-moz-platform: linux){
   #TabsToolbar .titlebar-buttonbox{
-    -moz-box-align: stretch !important;
+    -moz-box-align: stretch !important; /* Fx < 112 compatibility */
+    align-items: stretch !important;
   }
   #TabsToolbar > .titlebar-buttonbox-container > .titlebar-buttonbox > .titlebar-button{
     transform: scale(0.8);