Update flexbox support on Firefox

The flexbox pref in about:config is turned off in Firefox Beta 20, but turned on in Firefox Aurora 21.

See

>Our CSS3 flexbox support isn't quite production-ready, so I think we should keep flexbox support preffed off *in release builds* for the time being (but preffed on in nightly & aurora).

http://bugzil.la/841873
http://bugzil.la/841876