The feature is still not enabled by default, but it's close to being ready
Support for CSS3 Flexbox is proceeding as planned at Mozilla, the latest Firefox 19 nightlies offer support for the unprefixed property, meaning that Mozilla is confident its implementation is standard-compliant and that it's stable enough to lose the moz- prefix.The feature had already been built into Firefox 18 and work on it continues; however, it is still not enabled by default. Anyone wanting to test the support has to poke through about:config, search for "layout.css.flexbox.enabled" and set it to "true."
The stable Chrome already supports the latest CSS3 Flexbox draft spec by the W3C and Opera has just added it to its latest Opera 12.10 Beta builds.
With Opera 12.10 stable coming out by the end of the month in all likelihood, Firefox will be the last to add support, early in 2012 at the earliest.