While trying to get my Firefox add-ons and themes to update, install, or uninstall, I kept getting an error from Firefox saying the (after downloading the add-on) that the install file was not available - or something like this. It will show e.location.getItemLocation(e.id) is undefined in the error console under Tools->Error Console. I tried to do research a fix and found little documentation on this specific error, so I followed mozilla web links and this is how I came to resolve the problem.
I installed a backup utility to backup my settings, bookmarks, etc. (MozBackup - http://mozbackup.jasnapaka.com/). This will make sure we don't loose anything important in the process.
Then I used the utility to backup my profile named 'default' (This is the profile Firefox builds by default)
IMPORTANT: Close all instances of firefox (this means any processes as well)
Once, everything was backed up, I went to Start->Run (on the Windows XP Taskbar)
and entered 'firefox.exe -P'. This will only work if you installed firefox with an installer, if you downloaded a zip file of firefox, you will have to enter the full path to the firefox.exe (i.e. C:/)
This will open a simple program showing your default Firefox profile with the option to create, remove, etc. Create a new profile with whatever name you choose and close the program)
IMPORTANT: Again, check to make sure all instances of firefox are closed/stopped (this means any processes as well)
You will now have to manually copy over the files you want from your old profile to your new one. You will go to start->Run (just like last time, from your windows taskbar)
Enter '%APPDATA%' (no single quotes), this will open a directory with all your application data for programs on your computer, choose Mozilla->Firefox->profiles.
You should now see atleast 2 directories with unqiue id's followed by the profile name (i.e. 0843D.default or 0842D.newProfileName) Since there is a list and descriptions of these files, the best way is to follow the instructions mozilla gives for migrating data. The link I used is, http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox)
NOTE: When I transfered data, I checked firefox to make sure it wasn't having issues as it went. You may not want to copy everything over, I assumed my problem was caused by a bummed add-on and just manually reinstalled my add-ons, and only the ones I really needed.
If you are still having problems, you are welcome to see my sources and ask me questions. Thanks and good luck.
NOTE: Since this tutorial is written after fixing the problem, and I can't recreate the problem, please let me know if something isn't clear.
Migrating Data: http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox
Manually Profile Backup - http://kb.mozillazine.org/Profile_backup
Using MozBackup Utility - http://kb.mozillazine.org/MozBackup