It can also help if you delete the Firefox program folder (C:\Program Files\Mozilla Firefox) before

It can also help if you delete the Firefox program folder (C:\Program Files\Mozilla Firefox) before installing a new full Firefox copy.

See http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Clean_reinstall

It is important to delete the Firefox program folder to remove all the files and make sure that there are no problems with files that are locked in any way. You can skip the step to create a new profile, that is not necessary for this issue.