Mozilla is DRIVING ME CRAAAAAAAAAAAZY!!

Hey man I understand your frustation. Nothing sucks more than losing hours of trying to figure out what the shit is going on. I strongly suggest you uninstall mozilla, then do a search for every fucking file or folder with the file name Mozilla and Firefox. Do the same on the registry. After that, get the portable edition at www.portableapps.com.
extract it anywhere and use. Install your usual themes and extensions. If something goes wrong, delete the entire folder and reextract the archive and bam, instant clean install. The portable edition has all the required files and settings on the archive so with this we can isolate the problem. Also doesnt write to the registry or any other folder outside of where you extracted it to. Heck, it was meant to be run on a USB stick. Unlike the standard with installation one where it writes to several folders on your Documents and Settings profile. Try it and if you still get the error, possibly a kernel or background service is messing up with the firefox executable or flash or whatever.

Jam it back in, in the dark.