Try running Firefox once as an Administrator in Vista. If that doesn't work, , exit Firefox and try deleting the following files in your profile folder: '''extensions.rdf''', '''extensions.cache''', '''extensions.ini'''. See https://support.mozilla.com/en-US/kb/Profiles#How_to_find_your_profile for finding the folder, and http://kb.mozillazine.org/Unable_to_install_themes_or_extensions_-_Firefox#Corrupt_extension_files and [[Chrome registration failed]] for more information on causes of this problem.

Problem fixed! I use NoScript as a critical component to block out unwanted scum. All I had to do was re-download it after deleting those files and restart firefox. Thank you.