For some reason I can't log in to Stack Overflow. When I click "log in" no OpenID provider icons appear. If I log into meta and then go back to stackoverflow, I'm "half" logged in: I get the usual bar with my name, rep, etc at the top, but stackoverflow thinks I'm logged out when I try to answer questions, and clicking on the "stackexchange" icon in the top bar does absolutely nothing.
How do I fix this problem? I've tried clearing my cookies but that didn't help.
I'm using Firefox and have never had a problem logging in before today.