This is the new CSS Flexbox model -- using "display: flex", as opposed to the older "display: -moz-box" model. Also, in the spirit of avoiding vendor prefixes for experimental features, we'll be shipping this new flexbox implementation without a vendor prefix in development builds -- hence, "flex" instead of "-moz-flex". It'll be guarded by a hidden preference ("layout.css.flexbox.enabled") which we can toggle to make it available in release builds, too, once it's been sufficiently tested and had its kinks worked out.
Today's patch simply toggled that hidden preference to "on" in Nightly builds, and that change will be merged to Aurora (Firefox alpha) builds around January 6th, the next merge day. With some luck, this may continue to "ride the release trains" and be released as part of Firefox 20, but we'll evaluate its readiness when Firefox 20 is merged to the Beta channel and may toggle it off at that point (though anyone wishing to test flexbox would still be able to toggle the pref on for themselves).
So: please go forth and test the new flexbox support in nightlies, and file bugs for any issues you encounter! Eiji Kitamura's "CSS Flexbox Please" demo page is a fun place to start.
A few known issues:
- When printed, flex containers currently get truncated at page-boundaries (though I'm actively working on fixing that).
- The "order" property doesn't yet affect paint-order/layering (though it does control positioning) -- I've got a patch in hand that fixes this -- it's just awaiting review, and it'll hopefully land soon.
- The line-wrapping "multi-line" version of flexbox is not yet implemented. (That's the "flex-wrap" property and the shorthand "flex-flow" properties.) I'll hopefully get to this in the near term, but it's somewhat separate from the core flexbox functionality, so we opted to implement it separately.