Hi
Update: Firefox version 22 has just been released.
This means that once users have updated to the new version of Firefox, (normally happens within the first month), 90% of desktop browsers will support flexbox, and 99%+ of smartphone and tablet devices already do.
Tip: most layouts for IE9 or below, will only require the use of a few floats to provide an alternative layout, as all other css styles will still apply. In order to help with any site maintenance, I would recommend the link to the IE9 and below style sheets be placed inside an IECC.
Thanks,
PZ