Seems to work fine except for one thing. I can't connect to my KeePass database with the KeePassXC addon. My KeePassXC client doesn't see the Firefox flatpak browser either. I have a strong suspicion this has something to do with the flatpak sandboxing the browser. Whether it is or not, this is a killer bug for me. Therefore, I'm going back to the original Firefox and will wait until this flatpak is better polished.
Everything works as expected and the updates come fast.
They've finally put out a flatpak release of Firefox. Using a Flatpak version instead of the regular pre-installed Firefox, you'll get updates faster.