Sometimes you mistype preference name or simply wants to delete any value in about:config. There is no option to delete when you right click inside about:config entries. So to delete an entry, right-click it and select Reset. Then, restart Firefox. If the value you chose was originally part of the browser, it will go back to its default value. If the value was added by you, it will be deleted next time you start Firefox.
Another method is to look for prefs.js file in firefox profile directory. Before you edit that file, exit the firefox. Then open the file with text editor. Search for the entry you want to delete. When you find it, delete the line. Save the changes made to file, exit the text editor. Now you can start the firefox, and check the about:config entries.