In Firefox web browser, about:config provides a powerful way to configure and change the advanced preferences and settings of Firefox. Editing advanced preferences of Firefox through about:config page is easy and flexible, but it also prone to error, including possibility of creating a new preference name.

The new preference name, valid or invalid, may be created inadvertently or unintentionally. However, within about:config page, Firefox does not provide a way or option to delete and remove the preference.

Firefox can automatically delete invalid or erroneous preference keys that been inserted or added manually into Firefox. Firefox normally clear these invalid entries or name items on every restart. So if the preference name the been accidentally added is not valid and not supposedly to exist, try to close and exit from Firefox, and start again the Firefox. The preference name will be cleared from about:config page.

Tip: If the preference name is invalid, and does not been removed automatically after restarting Firefox, try to assign or set null (“”) as its value.

For some preference names, which may be valid and not been used by default, Firefox does not remove nor delete the items automatically. For such preference names, user can manually remove the preferences from the prefs.js file of Firefox. Here’s the step-by-step guide on how to delete preferences from Firefox through prefs.js.

  1. Close Firefox.
  2. Open Windows Explorer, and navigate to the following folder:

    %APPDATA%\Mozilla\Firefox\Profiles\

    Tip: Copy and paste to address bar, or launch through Start Search (Windows 7 and Vista) and Run command (Windows XP and 2000).

  3. Backup the prefs.js file located inside profile folder.

    Note: The file is located at the user-specific sub-folder of the Firefox profile’s folder.

  4. Open the original prefs.js in a text editor.
  5. Locate the line with the erroneous preference key name. Remove the whole line which starting with “user_pref” and ending with “;” to delete the preference away.
  6. Save the file, and run the Firefox again.

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