Are you the admin or not? Proxy error isn't a browser but a proxy site issue.

I am the only person using this computer, so I guess I"m the admin.

Then you got O/S issues happening.

Okay...and?

You can check the about:policies#active page in Firefox 63+ to see if there are policies active. It is also possible that there is a autoconfig.cfg file active in the Firefox program folder.

See also:

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

No policies active that I can find.

jansopran said

Okay...and?

Did you type the proxy into the url and see what you get back or did you also do a "ping" to see it says?

Sorry, I have no idea what that means.

Besides policies, another way to lock preferences is through an Autoconfig file.

Could you check for alien script files in your Firefox program folder? In particular, in these locations (varies for 32-bit / 64-bit):

• C:\Program Files\Mozilla Firefox\defaults\pref
• C:\Program Files (x86)\Mozilla Firefox\defaults\pref

Caution: Do not double-click script files! The default action for a script file it to execute as a Windows system script. (Typically this would just not work, but why risk it.)

Make sure Windows is showing hidden files: https://support.microsoft.com/en-us/help/14201/windows-show-hidden-files

A file named channel-prefs.js is normal. Any other file in this folder is suspicious and could be used to lock preferences. Remove any such files to a neutral location for further analysis at your leisure. You can rename the file with a .txt extension and examine it for the name of another file that contains the locking code.

More info:

• Customize Firefox using AutoConfig
• http://kb.mozillazine.org/Locking_preferences

Finally, I believe an extension might also be able to change proxy settings. You can view, disable, and often remove unwanted or unknown extensions on the Add-ons page. Either:

• Ctrl+Shift+a (Mac: Command+Shift+a)
• "3-bar" menu button (or Tools menu) > Add-ons
• type or paste about:addons in the address bar and press Enter/Return

In the left column of the Add-ons page, click Extensions.

Then cast a critical eye over the list on the right side. Any extensions Firefox installs for built-in features are hidden from this page, so everything listed here is your choice (and your responsibility) to manage. Anything suspicious or that you just do not remember installing or why? If in doubt, disable (or remove).

Any improvement?

Did you look for an autoconfig.js file in the Firefox program folder like I wrote above?

It is also possible that there is a autoconfig.cfg file active in the Firefox program folder.

The file can possibly be named different and is always specified via a .js file in the "/defaults/pref" folder in the Firefox program folder where the channel-prefs.js file is located like posted above.

Maybe also check in the Windows Registry whether GPO policies are present.

• HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Mozilla\Firefox\

Thanks to you all for your suggestions. I did type the Proxy into the Url, and it seems that this is a malicious software problem. I am investigating various ways of removing it.