Did it give a name to that file?

Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

You get this warning if there is a local-settings.js or autoconfig.js file in the "defaults\pref" folder where the channel-prefs.js file is located that specifies to use mozilla.cfg.

See Configuration:

• https://developer.mozilla.org/en-US/Firefox/Enterprise_deployment_before_60

If there are .js files present in the "defaults\pref" folder apart from the channel-prefs.js file that sets app.update.channel then open this file in a plain texteditor like Notepad (Mac: TextEdit) and please post its content. Also please post the content of the mozilla.cfg file.

There shouldn't be a mozilla.cfg file present unless you've created it yourself to initialize some prefs.

If you have this mozilla.cfg file then best is to do a clean reinstall and delete the Firefox program folder to make sure that all files are removed before reinstalling a fresh copy of Firefox.

• https://www.mozilla.org/en-US/firefox/all/

• http://kb.mozillazine.org/Preferences_not_saved#Other_solutions