It appears that Microsoft Store prevents desktop programs to be installed if they are also listed in the Microsoft Store and I am not on an S version.

You should still be able to install on Microsoft store if you prefer that. https://support.mozilla.org/en-US/kb/download-firefox-windows-microsoft-store

https://www.mozilla.org/firefox/all/desktop-release/ https://support.mozilla.org/en-US/kb/how-install-firefox-windows

Yes that is what I am using right now. It's 32 bit and has no .exe file which I could link to my VPN so only that programe uses the VPN and none of the others.