I have a 2011 MacBook Pro, which is running Firefox 20.0 and OSX 10.8, and StackOverflow.com is not rendering correctly. It seems to only do this with SO, Meta is working fine.
closed as too localized by Bart, Aaron Bertrand, hims056, animuson♦, Lucifer Apr 16 '13 at 4:25
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I have this problem occasionally when I'm connected to a very slow VPN (and in my case I need to route traffic over that network to get VPN-specific DNS to work correctly). I suspect you experienced some type of network hiccup; this is not a rendering problem, it is that your browser did not (completely) load the CSS to apply proper styling. I just gave Aurora a shot and all of the sites I frequent render just fine.