Could you please elaborate, which software is usable on mobile Linux except for Firefox? I've seen multiple people using mobile Linux, and they were using Firefox and webapps for everything, no exceptions.

Checking Flathub should give you some idea: https://flathub.org/en/apps/collection/mobile/1

There are more, not every application that works fine has metadata filled up (and not everything is on Flathub either).

I do use some webapps, but with Epiphany rather than Firefox.

I can use most native GNU/Linux apps on my Librem 5 like gnome-calculator, gnome-calender, gnome-weather etc. I can run Android apps via Waydroid. F-Droid works fine, too. Its default app store (https://software.pureos.net/categories) provides things like music players, OTP app, and games. Flatpak works, too.

See also: https://linuxphoneapps.org/