Have you tried to redownload, and reinstall it?
It could have been a corrupt download.
Uninstall your current version from Add/Remove programs.
Then go to C: / Program files, and delete the Mozilla Firefox folder.
To completely uninstall Firefox, you will need to delete your Profile folder.
This file is not removed when you uninstall Firefox from Add/Remove.
It will retain all of your previous settings, and re-apply them to your new install, if not deleted.
Set windows to show hidden files,How to see hidden files in Windows
then navigate to:
C:\ Documents and Settings \ user name \ Application Data \ Mozilla \ Firefox \ Profiles
Delete this folder, and then try reinstalling Firefox with a fresh download.