What OS are you using?
My lappie with FF8 on Win7 is okay thus far, and was for FF5 & FF7 too.
Or, have you other programs (anti-spyware, etc) with over-ride settings?
IIRC, SpywareBlaster (and Spybot S&D) have anti-tamper settings, so these could be "correcting" anything FF tries to overlay.
I did have an add-on theme for a while on FF5 which was giving grief, so ditched that (can't recall which though - wasn't Lavafox but was similar) and all returned to normal.