It should only appear once (the first session after you have updated
Firefox). If it continues to appear in every subsequent session, try this:
1. Close Firefox.
2. Click the Windows Start button, and select Run....
3. Type in %APPDATA%\Mozilla\Firefox\Profiles\ then click OK. Windows
Explorer will open a folder containing your profiles.
4. Double-click the profile folder (e.g. the xxxxxxxx.default folder).
5. Delete the file: Invalidprefs.js
--
Chris Ilias <http://ilias.ca>
List-owner: support-firefox, support-thunderbird, test-multimedia
Keeper of the Knowledge Base: <https://support.mozilla.com/kb/>