Previously this just used --toolbar-bgcolor which may have transparency
leading to poor readability. Similar results might still happen if theme
has poor text-background contrast, but now the result should be less
unpredicatable.
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.
Before this change, the status would show fully, so on narrow windows
the input controls and close button would be pushed out of the window.
Now the status text will instead get clipped.
MrOtherGuy