This allows this style to work with several other styles such as
tabs_on_bottom.css
This patch also introduces a pref with which the side of the vertical
strip can be changed - the default is now on left
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.
MrOtherGuy