Note, this only affects Firefox 136+ with its new audio button.
This patch also restricts the style to horizontal tabs because Vertical
tabs can be collapsed with built-in feature.
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.
