Well, tweaking and fiddling with Firefox can be done - but mebbe it'd be easier to just uninstall your present Firefox (through Add/Remove Programs), then download a fresh copy and reinstall it, setting everything to default to start with when you do the fresh install. Sometimes its better just to do away with a problem than to try to fix it.
If you go that way, I suggest that after you uninstall your current Firefox, you use
CCleaner to clean up after the uninstall; use both the standard cleanup and the "Issues" cleanup -
See This. When that's been done, reboot, then locate and install your fresh copy of Firefox and lemme know what happens.