This problem is very common to Mozilla Firefox users. Many times when you try to install a new extension or theme in Firefox, you get following error message:
Unexpected Installation Error -203
Even existing add-ons also don't work. Its a very annoying problem and after getting a few e-mails asking for solution of this problem, today I decided to write a small tutorial to fix it.
So here we start the tutorial:
First make sure you are not using Zone Alarm or CA firewalls as both these firewalls are known to cause this problem.
If you are using any of them, disable it by using its system tray icon and then try to install the extension or theme. Now it should install without any problem. After the installation, you can again enable the firewall.
Most of the times, this problem occurs when you update the Firefox version installed in your system. If you have recently updated Firefox version, then delete following 3 files from your Mozilla Firefox Profiles folder:
These files will be re-created automatically when you start Firefox next time and your problem will be fixed.
NOTE: To find the location of Profiles folder, visit following link:
If the above solution doesn't work, then its a problem of corrupt profile folder. Sometimes your existing Firefox profile folder gets corrupted and causes this problem. You'll need to create a new default profile to fix this problem. But it might remove all your installed add-ons.
1. Open Firefox Profiles folder as mentioned in the link given in METHOD B. Don't go to the "xxxxxxxx.default" folder, go one folder back and you'll see a file "profiles.ini" in the same folder where "Profiles" directory exists. Either rename or delete the "profiles.ini" file.
2. Now double-click on "Profiles" folder and delete "xxxxxxxx.default" folder. Make sure to take its backup if you want to save your bookmarks, cookies and passwords later.
3. Now run Firefox again and it'll re-create the default profile folder and will work like you installed it for the first time. It'll fix the problem.
PS: To restore your bookmarks, cookies and passwords copy "bookmarks.html", "cookies.txt", "history.dat", "key3.db" and "signons2.txt" files from the backed-up profile folder to newly created profile folder.
Firefox 3 users will need to copy "cookies.sqlite", "places.sqlite", "key3.db" and "signons3.txt" files as Firefox 3 uses these new files instead of those old files.