This is a great workaround after the update mentioned in Bug 1664477. Thanks a lot!

=Edit=
By the way, despite that the extension is a bit buggy, I still gave 5 stars because it's currently the ONLY solution. This is the way I use the extension:

=How to prepare it=
1.- Install the extension.
2.- Press the extension button active it (as seen in the screenshots).
3.- DISABLE THE EXTENSION! The idea is that the on/off button of the screenshot must be on an active state (green) before disabling, so it resumes when it gets enabled.
=How to use it=
1.- When you want to get a clean window, open a new window and go to the page you want.
2.- In that new window, press Ctrl+Shift+A or manually go to the Addons manager.
3.- Enable the extension. This action will turn the first opened tab of the current window into a new clean window.

(this works for me with Firefox 84.0.2 under Manjaro Linux, but I don't see why it shouldn't work on Windows or MacOS)