It only works in Chrome because as there's a *vertical* scrollbar. If there's an *horizontal* scrollbar bar but NO vertical scrollbar, it starts behaving exactly the same as Firefox here, unless you do that: $('.post code').css('flex', 'none'); (This hack is signed yours truly. Of course, don't execute it if there's a vertical scrollbar or no horizontal scrollbar on that element. As a result, the code in Wedge is more complicated than that.)
I still don't know why both browsers fail at that, really. Flexbox is supposed to be the next big thing.
As a reminder, iOS 7 also supports flexbox, and also fails with it, this time with suggested flex widths.