This fix from the Windows tech support function fixed the issu for me.
Unexpected installation error -203
This error message can be caused by corrupted configuration files on your computer.
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 File and then select the Exit menu item.
2. Go to your profile folder.
3. Delete the following files:
* extensions.ini
* extensions.cache
* extensions.rdf
4. Restart Firefox.
**How to find your profile
Windows Vista
1. Click the Windows Start button, and type %APPDATA%\Mozilla\Firefox\Profiles\ in the Start Search box near the bottom of the Start menu, without pressing Enter. A list of profiles will appear in the top left of the Start menu.
2. Click on any of the profiles (e.g. xxxxxxxx.default) to open it with Windows Explorer.
On Windows 2000 and XP
1. Click the Windows Start button, and select Run. Type in %APPDATA%\Mozilla\Firefox\Profiles\ then click OK. Windows Explorer opens a folder containing your profiles.
2. Double-click the profile folder you wish to open (e.g. the xxxxxxxx.default folder).**
Soon as firefox had restarted,it had all the addons/extensions i previously had & was as it should be!!
