First, go to http://www.avg.com/us-en/avg-rescue-cd-download and download the Rescue CD. Follow the instructions to create the disc/usb. Next, go to the Firefox web site and download the install files. Do NOT go to any other site to get the files. In Control Panel, choose to uninstall Firefox. Once uninstall is complete, place AVG Rescue disc in drive and reboot, making sure the PC boots from the CD/USB. Once the Rescue is done, reboot and install Firefox. Make sure you have the latest antivirus software and firewall running. AVG, Norton, and Panda are among the best. I had this problem when I clicked on link for some no-name antivirus out of England.It took over the system and would not let me do anything. I could only get online by booting into safe mode with internet. Good luck.

Thanks for the advice. I had the problem on a work computer with a crapped-out optical drive. It was set up with multiple user accounts. I was able to boot in safe mode in the administrative account. I uninstalled firefox and ran AVG. Rebooted and it was fine. Ran Malwarebytes and Spybot, they came up clean. It appeared to have changed my proxy settings in Opera and IE though. I uninstalled Opera and don't really mess with IE anyway.

Thanks for the quick and helpful feedback.