I would download a fresh Firefox.
I'd set IE back to all default settings.
I'd then uninstall all browsers including IE via the control panel.
Then I'd run ccleaner, both the cleaner & registry cleaner.
Then I'd install the downloaded Firefox refusing any options to use IE settings.
I can't remember if those install options exist in Firefox but I'd be looking for an untainted install.
Then I'd test Firefox and if it worked I'd enable IE & test it.
Great thread Howzer.