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/