Click the Firefox button at the top left, then click the Help menu and select Troubleshooting Information from the submenu. If you don't have a Firefox button, click the Help menu at the top and select Troubleshooting Information from the menu. Go to Open Profile Directory (or something similar) and delete a file named user.js

I did as you suggested, but there is no file named user.js in the Profile Directory.

Thanks

This happens when an add-on needs to be unpacked in order to work properly and this hasn't happened.

• https://developer.mozilla.org/En/Updating_extensions_for_Firefox_4.0#XPI_unpacking

Create a new profile:

1. Hold Alt + F2 or Super (windows key + R)
2. type "firefox --profile manager"
3. a window will show, add a new profile
4. make sure you uncheck Dont ask on startup
5. See if issues resolves (go to the profile you just created)

http://kb.mozillazine.org/Profile_Manager

Sorry, but this looks like it is for someone who has more Linux knowledge than I do, maybe for a developer.

Thanks

I edited it.