I believe the immutable distros you’re referring to are Fedora Silverblue and Fedora Kinoite.
Yes. I do have some applications installed as flatpak. What’s the problem?
What issues do you face with Flatpak?
I don’t get it. Do you have two versions of Firefox installed?
I’ve been using Aegis Authenticator for about two years now . It is free and open-source, and works as expected.