Go to your profile folder and search for user.js file. Does it contain this user agent?

there is no user.js file. prefs.js only

You can check what .js files are present in the "defaults\pref" folder in the Firefox program folder. The channel-prefs.js file that sets app.update.channel is a legitimate file in this folder, any other .js file is suspicious. You can open this suspect file in a plain text editor like Notepad (Mac: TextEdit) and please post its content to see whether that allows to identify what software created the file. Also please post the content of the autoconfig.cfg file referenced in this autoconfig.js file.

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

If you have this autoconfig.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 freshly downloaded copy of Firefox.

"C:\Program Files\Mozilla Firefox\defaults\pref\local-settings.js" pref("general.config.obscure_value", 0); pref("general.config.filename", "mozilla.cfg");

autoconfig.cfg/js is absent

Thank you, cor-el. The mozilla.cfg file does indeed contain pref("general.useragent.override"... Apparently added by the employer