I encountered the above problem after I have upgraded to Firefox 3.5x. After disabling all plug-ins and doing a clean installation of Firefox several times, I finally found the problem - my free version of AVG. To be more specific, the problem is with AVG’s SafeSearch extension.
There is a discussion thread here and the problem is also documented in Mozilla.
The simple solution is to uninstall the old version of AVG and install the new version.
