I opened and closed Firefox a few times, and I think my cache gets deleted since I clear history on close. I didn't get any errors, it just stayed on "Joining game" for ages but nothing would happen. I was able to join one game but everything was messed up and lagging and I couldn't do anything. I checked processes to see if there was some "ghost" BF3.exe hanging around (which has happened before), but there was nothing, so no idea why it wouldn't join the games. I turned the firewall off, though it would be nice if the game would tell me if the ports were blocked.
I never managed to get Firefox to launch BF3 correctly, so downloaded Chrome and it works perfectly every time. Now use Chrome for BF3 and Firefox for general browsing.