So you know about addons aka extensions/themes and tried them. You may have found one you no longer want or need. You can either disable or completely uninstall them easily. Just go to the “Tools” menu and select “Addons”. From there browse to the one you no longer want, click on it and press the “Disable” or “Uninstall” button.
I’ve seen complaints that it doesn’t appear in “Add Programs” on Windows. That’s because it’s not installed on Windows, but in Firefox. Uninstalls are still easy and painless.