Firefox hijacked??

I run Firefox 0.9.2.
System shows clean on adaware, spybot, cwshredder etc. No untoward BHOs. I can access the internet perfectly OK through IE.
Firefox has four homepages. The first three keep getting changed to about:blank after a reboot. The fourth is OK. The three are google, the bbc and bbc local. Ive tried changing them, but they still wont resolve host, and after reboot come up in options as about:blank
Firewall is configured OK - ive even tried turning it off.
Has Firefox been hijacked, or is it something simple ive overlooked?