There are multiple causes to this error:
- corrupted plug-in
- corrupted Firefox file
- corrupted profile
- virus infection
Firefox offers the possibility to start the browser in Safe Mode. To do this, go to Start > All Programs > Mozilla Firefox > Safe Mode. Start the browser in this mode and delete all the plug-ins and add-ons then start it again. If the error still persists, perform a fresh installation of Firefox and download the newest version from its official website.