Found the solution, at least for me. If you have tried all the other given fixes, and you still have trouble like I did, it may be PeerGuardian or your firewall/antivirus program.
If it is PeerGuardian, check «allow all connections to destination port 80» in the preferences window.
The place where I found this solution is located here:
http://forums.mozillazine.org/viewtopic.php?t=107957