Mozilla publishes firefox on Flathub and anybody can install it from there. After, I'm not sure why they don't advertise it, most apps distributed this way just have a button that do so.

Fedora has its own repo, they manage it, i don't see the problem there. After it doesn't prevent adding others like flathub and the experience from a user point of view is the same.

You can also provide a flatpak ref file that user can use to install.

Signing app doesn't means much appart that someone paid for that and went through a process IMO, there's not much value to it from the user pov, especially when the first thing a Windows user learns is to ignore signature warnings.

Have you tried using flatpak?