Is it just me, or did the layout on the questions page break with Firefox 20 for other people too? And if so, is it Firefox's fault?
I found out what triggered it: Selecting a bigger font size. I have View → Zoom → Zoom text only enabled, and when I zoom in, the font size increases while other sizes stay the same, and then the layout breaks, as the “ASK QUESTION” item floats to the line below.
I think this is most likely a CSS bug on the site, and not Firefox's fault.