Williamo’s Blog.

I first came across this error when trying to install the Read It Later extension, then saw it again when attempting to install Live Writerfox, and discovered that it wasn’t a problem with either extension, but a problem with corrupt extension files (or potentially a corrupt profile).

From Mozillazine.org:

Installing themes or extensions may fail with the message, Unexpected installation error -203. (If this occurs after a Software Update you may find that add-ons that were previously installed also do not work.) Follow the above directions to remove corrupt extension files. [14] If the problem is not resolved and you use ZoneAlarm or CA Firewall, disable it temporarily (enable Windows Firewall in the interim) to allow the installation. [15] If the problem persists, your profile may be corrupt (bug 361358). Use the Profile Manager to create a new profile [16] or create a new default profile [17] and then transfer your data to the newly-created profile.

(http://kb.mozillazine.org/Unable_to_install_themes_or_extensions_-_Firefox#Unexpected_installation_error_-203)

And here’s the fix:

Files that store information about the extensions you have installed can sometimes be corrupted. Deleting them can fix a number of extension issues; they will be regenerated the next time you start Firefox. [12][13]. Exit Firefox completely, then open your Firefox profile folder (read the linked article for its location) and delete* these three files:

extensions.ini

extensions.cache

extensions.rdf

• Note : Although unwanted files/folders can be deleted, renaming them is generally considered a safer option. This achieves the same result, but allows the user to retrieve any possibly required information from them later on. In the case above, rename the above 3 files as , ‘extensionsOLD.ini’, ‘extensionsOLD.cache’, ‘extensionsOLD.rdf’

(http://kb.mozillazine.org/Unable_to_install_themes_or_extensions_-_Firefox#Corrupt_extension_files)

If that still doesn’t fix the issue, your profile may be corrupt (as mentioned above), and you’ll find a fix at the link below, which involves recreating your profile from scratch:

https://bugzilla.mozilla.org/show_bug.cgi?id=361358

Hope this helps you out…it was quite frustrating for me.