It's a bit more complex: there are actually 2 flexbox specs with different syntaxes, one from 2009 (now deprecated) and one from 2011[0]. Firefox has supported the old syntax circa Firefox 2 (Safari from 3.1, Chrome from the first version).
Firefox never supported the old flexbox. It has XUL boxes (dating back to 1998 or so), which the old flexbox was syntactically, and somewhat behaviorally based on. But they predate those drafts by 10 years.
I'm not sure why you're asserting that when you're completely wrong. Not only did Firefox support the old flexbox, it still does as of Firefox 22: [0] still works and uses the old (and prefixed) flexbox syntax, and said old (and prefixed) syntax remains documented on MDN with a note that it's deprecated[1]
My point is that the syntax in question is used for XUL boxes, and has been for going on 15 years. It's not used for any sort of implementation of the old flexbox draft.
The old flexbox draft is similar to XUL boxes in some ways, and quite different in other ways. Trying to use XUL boxes and expecting the old-flexbox behavior as a result will land you in trouble, as happens with depressing regularity when people use "display: -moz-box".
The MDN documentation is sadly wrong, because it ignores the fact that the properties _look_ similar but have totally different behavior. Thank you for pointing that out; I'll make sure it gets fixed.
This is about the new syntax and current CR.
[0] http://css-tricks.com/old-flexbox-and-new-flexbox/