Uninstall a theme
Normally, the public themes are tested and therefore, you shouldn't have any problems. However, if some theme messed up with your computer, here's how to uninstall it.Try those steps before, then check out the complicated way if this doesn't work.
1) In the Firefox menu, click on Tools, then on Add-ons.
2) Click on the theme you want to uninstall.
3) Click on Uninstall.
It is possible that the steps above didn't work, but that happens very rarely. Follow those instructions if this is your case.
1) In your list of programs, find Mozilla Firefox.
2) You should see two items: Mozilla Firefox and Mozilla Firefox (Safe Mode). Select the second item.
3) Click on Tools, then on Add-Ons.
4) Click on the theme that causes you problems.
5) Click on Uninstall.