Did you check tools>options>general tab>check if FF is your default browser.
If this is a hijack issue,click HERE
NOTE:use with caution.
I was using firefox and than switched to IE. When I tryed to get back to firefox it said it was already running and to restart my computer. I scanned with spysweeper,ad-aware and than yahoo anti-spy. Yahoo anti-spy found Pest Control had hi-jacked my firefox browser. I was downloading flashplayer 8 from Adobe on IE. What is pest control and did it come from the flashplayer download. Surprized that spysweeper or ad-aware didn't catch it. Has this happened to anyone before on firefox being hi-jacked? My OS WindowsXP SP2. By the way I did not restart my computer.