As Green Granite says.
Something is corrupted in your profile settings somewhere, and those will reload from installation to installation. Find your address book and bookmarks, and save those somewhere else. Then delete the Firefox profile, uninstall, reinstall, and reimport the bookmarks etc. All should be well.
You'll also need to reinstall any add-ins (NoScript, AdBlock Plus and Web Of Trust are pretty well essential, others are "to taste").