Remove Erroneous Firefox Configuration Entry (Windows 7)
If you entered an erroneous entry into the about:config database of Mozilla Firefox, you will have noticed that there is no option in the browser itself that enables you to remove that entry. Instead, find the config file in your desktop explorer:
YourUsername has to be replaced with the name of the user you’re logged in with and weirdstring is a string consisting to mainly numbers which represents your Firefox profile. Edit the prefs file with notepad, track down the entry you want to delete and erase the full line.