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.

Table Of Contents

• Corrupt extension files
• Security software

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:
   • (Firefox 3.0 and 3.5): See How to find your profile
   1. (Firefox 3.6): At the top of the Firefox windowOn the menu bar, click on the Help menu and select Troubleshooting Information.... The Troubleshooting Information tab will open.
   2. Under Application Basics, click on Open Containing FolderShow in Finder.
2. From the menu at the top of the Firefox windowbar, select FileFirefox and then select the ExitQuit FirefoxQuit menu item.
3. Delete the following files:
   • extensions.ini
   • extensions.cache
   • extensions.rdf
4. 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 Firewalls article.