@mokazemi
You may want to take a look at the @tuxedocomputers repositories, that's how I get my #Firefox.
(Full disclosure: I bought a machine with it preinstalled) @JTskulk
Why use flatpak when you can just use deb repositories?
I listed one way that I know works because that’s how I use Firefox on my Steam Deck (which in turn uses Plasma 5 as desktop, same as Kubuntu). If a deb package from some PPA works just as well: great.