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_toolbar.css

@@ -39,8 +39,10 @@ Window controls will be all wrong without it
 }
 
 :root[tabsintitlebar] #toolbar-menubar[autohide="true"] #main-menubar{
-  -moz-box-flex: 1;
-  -moz-box-align: stretch;
+  -moz-box-flex: 1; /* Fx < 112 compatibility */
+  flex-grow: 1;
+  -moz-box-align: stretch; /* Fx < 112 compatibility */
+  align-items: stretch;
   background-color: var(--toolbar-bgcolor,--toolbar-non-lwt-bgcolor);
   background-clip: padding-box;
   border-right: 30px solid transparent;