If you check the network.proxy prefs on the about:config page, do some have a lock icon at the far left ?

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 other .js file in a plain text editor like Notepad and check what autoconfig.cfg file is referenced in the autoconfig.js file. There shouldn't be a autoconfig.cfg file present unless you've created it yourself to initialize some prefs.

• pref("general.config.filename", "autoconfig.cfg");

See also:

• https://support.mozilla.org/en-US/kb/customizing-firefox-using-autoconfig

Yes. There is only one setting that has a lock icon and that is network.proxy.type. See attached screenshot. If this is the problem, how do I edit that setting?