Once you have Flatpak setup you can run sudo flatpak install firefox

I already have Flatpak installed and it has the same problem as Firefox ESR (which comes with Debian by default, if my memory serves me correctly)

Also, is there a reason you are using Debian? If that’s what you want then that is fine but it isn’t something people use for the new packages.

I have used Debian-based distros in the past like Mint and Ubuntu so I wanted to use Debian itself out of curiosity.