I had a problem when Firefox upgraded itself from 12 to 13, namely that certain sites wouldn't open, Gmail and Google Groups, Facebook and Amazon. Same in safe mode. I did a search and found that quite a few people were having the same problem. I went back to 12 and disabled automatic update and waited for a fix to become known. Today I saw on the Mozilla forums that it seems to affect Windows 64 bit systems, and the problem happens if SPDY is enabled. It was disabled by default in previous versions, and FF 13 switched to SPDY enabled by default. Thus encouraged, I re-upgraded to 13 and did this: went to about:config and toggled network.http.spdy.enabled from TRUE to FALSE, then restarted Firefox. Now it's fine. I read that some people needed to toggle network.http.keep-alive to false, too.
