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:

   At the top of the Firefox window, click on the Firefox button, go over to the Help menuOn the menu bar, click on the Help menuAt the top of the Firefox window, click on the Help menu and select Troubleshooting Information. The Troubleshooting Information tab will open.

2. Under the Application Basics section, click on Open Containing FolderShow FolderShow in FinderOpen Directory. A window with your profile filesfolder will open.
Note: If you are unable to open or use Fire​fox, follow the instructions in Finding your profile without opening Firefox.
3. At the top of the Firefox window, click on the Firefox button and then select ExitAt the top of the Firefox window, click on the File menu and then select ExitOn the menu bar, click on the Firefox menu and select Quit FirefoxAt the top of the Firefox window, click on the File menu and select Quit
4. 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.