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/tabs_below_content.css

@@ -38,7 +38,7 @@ linux_gtk_window_control_patch.css
 /* Move tabs below content */
 #main-window > body::after{
   content: "";
-  display: -moz-box;
+  display: flex;
 }
 #main-window:not([inFullscreen]) > body::after{
   margin-bottom: calc(var(--tab-min-height) + 2 * var(--tab-block-margin,0px));