Totally agree with Phantom010... Please do not use ANY registry cleaners on your computer. You may get by with once, twice or any number of times, but it WILL bite you eventually.
I suggest you reinstall FF. If you are not currently running v3.5.2, you can simply update FF and that should suffice as a reinstall. Otherwise, I would uninstall and reinstall FF.
What Add Ons do you have active? If you suspect an Add On is the problem, they are simple to uninstall one at a time to see if one of them changes things. Perhaps this should be the first thing you should do before attempting a reinstall or update.
Raybro