I've been thinking about this one, one question, have you updated to Firefox 1.5? I don't know the significance of that yet if you have. The strange thing is the extensions are held in Firefoxes Profile Folder, this is the location of it in XP:
C:\Documents and Settings\[User Name]\Application Data\Mozilla\Firefox\Profiles If you have a look in there for the extensions folder and see if there is any clue, like if you can recognise any of the files that might relate to the extensions that should be installed. If they are not in there then Firefox must be picking up the extensions from somewhere else. If you come back and lets us know your findings.
OK, if all the extensions are present, then remove Firefox from add/remove programs and reinstall it, it should look for the profile folder and reload all the extensions back, make sure you back up your bookmarks by exporting them somewhere you can find them, just in case they are deleted by accident.
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