If you try to install an extension or theme, and receive the error message "Unexpected installation error -203", it is usually a result of extension configuration in your Firefox profile being corrupt or security software preventing those files from being updated.

Corrupt extension files

Deleting these files will make Firefox recreate them (removing any corruption) the next time it starts. You will not lose your extensions or their settings by deleting these files:

1. Open your profile folder:

   • Click the menu button , click Help and select More Troubleshooting Information.From the Help menu, select More Troubleshooting Information. The Troubleshooting Information tab will open.
   • Under the Application Basics section next to Profile FolderProfile Directory, click Open FolderShow in FinderOpen Directory. A window will open that contains your profile folder.Your profile folder will open.Your profile folder will open.
   Note: If you are unable to open or use Firefox, follow the instructions in Finding your profile without opening Firefox.

2. Click the Firefox menu and select Exit.Click the Firefox menu at the top of the screen and select Quit Firefox.Click the Firefox menu and select Quit.
3. Delete the following files:

• • extensions.ini
  • extensions.cache
  • extensions.rdf

1. Restart Firefox.

Security software

Make sure your Internet security programs (including antivirus, anti-spyware, firewalls, and privacy programs) are set to allow Firefox. For configuration instructions for several specific programs, see the Configure firewalls so that Firefox can access the Internet article.