Enter about:config in the address bar and check the value of general.config.filename pref.

See also Customizing Firefox Using AutoConfig.

The value of general.config.filename is mozilla.cfg

Is this the expected value?

The preference "general.config.filename" only exists if an Autoconfig file was set up. This could be done by an IT department on a work computer, or by some other software you installed on your system -- sometimes security software will do it, sometimes malware will do it -- or by you.

Try to remove it, or find the mozilla.cfg file.

Try to remove it, or find the mozilla.cfg file.

I was unable to find the mozilla.cfg file. I removed the value from general.config.filename and the message still pops up when opening Firefox.

The preference "general.config.filename" only exists if an Autoconfig file was set up. This could be done by an IT department on a work computer, or by some other software you installed on your system -- sometimes security software will do it, sometimes malware will do it -- or by you.

This is my personal computer so no IT department. I don't think I did it unless it can be done by accident. I use Windows built in security. When you say security software are you talking about third party software? Scanning with Windows Security does not find any threats. Is there a better option for scanning for malware?

Try uninstalling Firefox, download a fresh copy and reinstall. The default uninstall process will not clear your settings.

Along the lines zeroknight suggested, and as noted in the article linked earlier by TyDraniu, the files involved in Autoconfig are in the program folder, typically

C:\Program Files\Mozilla Firefox

(sometimes C:\Program Files (x86)\Mozilla Firefox)

You can remove the file that creates the general.config.filename preference and see whether that is enough to stop the messages. You probably also need to clear it on the about:config screen.

If you have the usual install location, the file would be in this folder:

C:\Program Files\Mozilla Firefox\defaults\pref

Normally there is only one file here, named channel-prefs.js, which is a normal part of Firefox. Any other file here is potentially creating this problem because Firefox will read it at startup. You could delete the extra file(s), or move it/them to some other folder for further investigation.

Note: If your Windows is not configured to show common file extensions, the .js might not appear. To see full file names including extensions, check the steps in this article: https://www.bleepingcomputer.com/tutorials/how-to-show-file-extensions-in-windows/

Along the lines zeroknight suggested, and as noted in the article linked earlier by TyDraniu, the files involved in Autoconfig are in the program folder, typically C:\Program Files\Mozilla Firefox (sometimes C:\Program Files (x86)\Mozilla Firefox) You can remove the file that creates the general.config.filename preference and see whether that is enough to stop the messages. You probably also need to clear it on the about:config screen. If you have the usual install location, the file would be in this folder: C:\Program Files\Mozilla Firefox\defaults\pref Normally there is only one file here, named channel-prefs.js, which is a normal part of Firefox. Any other file here is potentially creating this problem because Firefox will read it at startup. You could delete the extra file(s), or move it/them to some other folder for further investigation. Note: If your Windows is not configured to show common file extensions, the .js might not appear. To see full file names including extensions, check the steps in this article: https://www.bleepingcomputer.com/tutorials/how-to-show-file-extensions-in-windows/

Ah ha. There was an extra file in the defaults\pref folder. It was called antibeacon.js. I removed that and it's working as expected.

Thanks for the help.

Since your install folder has been tampered with, you need to uninstall Firefox and reinstall a fresh copy, then do a full system scan for malware to ensure it is clean.

Make sure you remove the Firefox installation folder after uninstalling to remove any leftover file/folder.