If this proxy does not work, go to settings page and allow it to access private windows. That is a must for setting proxy for extensions.

Input your custom manual proxy server. Feel free to make a XSS attack. If you succeed to do that, report a bug to me, or maybe mozilla!

After input, re-click the button in popup in toolbar to activate it! I bet you don't change this frequently so it's not a pain.

Do at your own risk if you input an invalid url!