userChrome/tabmenu: Add note to a declaration obsoleted in FF 113 (MR 31)

Firefox 113 changes from 'display: -moz-box' to 'display: flex', so the
-moz-box-flex property should be changed to flex-grow. However, that
breaks the tab menu, and it doesn't seem to be necessary anyway.

src/userChrome/tabmenu.css

@@ -56,7 +56,8 @@
         max-height: 300px !important;
         /* When messing around with tabs, it gets into a state where it does
          * not use the whole height anymore, it becomes a tiny window. Removing
-         * this attribute fixes it. */
+         * this attribute fixes it. Since FF 113, this no longer has any
+         * effect, but it doesn't seem to be necessary either. */
         -moz-box-flex: initial !important;
     }