http://kb.mozillazine.org/Firewalls

Please mark "Solved" one answer that will best help others with a similar problem -- hope this was it.

I had the same problem and was pulling my hair out. I posted here and some nice offered me 3 options, the two without success however, the third option did the trick.

It seems that my McAfee Total Protection Software was the culprit. I went in to the firewall settings in McAfee and set Mozilla Firefox to full from blocked. That did the trick and now it is working like a champ! Thanks to the people who share there ideas. I must admit that I wasn't a happy camper. I thought it was a Firefox problem but it was McAfee trying to protect me from the fox. I still think they should warn you that you security software might give you a problem after the update.

Thanks again!