Tip: Unexpected installation error -203 (Add-ons)

I’ve been seeing several forum members on Go Firefox! getting the error:

Unexpected installation error -203

When attempting to install extensions and/or themes. In most cases these members have recently upgraded from Firefox 2 to Firefox 3 on the same profile. This error message can be caused by either:

  • security software, which may be preventing these files from being updated
  • corrupted configuration files on your computer

99% of the time it is going to be caused by corrupted configuration files on your computer. To fix this problem follow these steps:

  1. Close Firefox
  2. Locate your Profile Folder
  3. Delete the following files from your profile folder:
    • extensions.ini
    • extensions.cache
    • extensions.rdf
  4. Restart Firefox.

When Firefox restarts it will automatically rebuild the 3 files in step #3 above and should allow you to be able to install add-ons again.