http://support.mozilla.com/no/kb/Una...tion_error_203

Unexpected installation error -203

This error message can be caused by either:

* corrupted configuration files on your computer. To resolve, see Corrupt extension files below.
* security software, which may be preventing these files from being updated

Corrupt extension files

Corrupt extension configuration files can cause problems when trying to install an extension. Deleting these files will make Firefox recreate them (removing any corruption) the next time it starts. You won't lose your extensions or their settings by deleting these files.

1. From the menu at the top of the Firefox windowbar, select FileFirefox and then select the ExitQuit FirefoxQuit menu item.
2. Go to your profile folder.
3. Delete the following files:
* extensions.ini
* extensions.cache
* extensions.rdf
4. Restart Firefox.