I am currently using Linux Mint 14 with Firefox 21. I am trying to add new themes and add ons, but I keep getting the message "firefox cannot modify the needed file in linux". How can I correct this?
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.
This happens when an add-on needs to be unpacked in order to work properly and this hasn't happened.
Create a new profile:
Sorry, but this looks like it is for someone who has more Linux knowledge than I do, maybe for a developer.