I added this edit above. Pasting here in case you are curious. Cheers.
EDIT: This thread motivated me to try and fix this issue. Installing Firefox using rpm-ostree worked. I expected it would, though I am still hoping to figure this out using the Flatpak version at some point. I also tried using Distrobox/Box Buddy to create a Fedora 40 box and install Firefox there. That version of Firefox couldn’t even see my NAS at all (unlike the Flatpak which could see my NAS but couldn’t upload files from the NAS to Proton). This was my first time ever using Distrobox. I thought it was super cool to see it in action and get a working Firefox, even though I couldn’t use it to access my NAS as hoped.
I vote the same, but I’d suggest a uBlue spin of the Fedora Atomic desktops. They have better defaults (all batteries included, as they say) and are easier to use overall IMHO. Bluefin and Bazzite are both great options, and both offer KDE and Gnome variants.
https://universal-blue.org/
Edit: TIL the KDE version of Bluefin is called Aurora.
BTW, uBlue is getting some big recognition lately. They have been on the Fedora Podcast (official) and Framework Laptops has official instructions on their website for installing Bluefin and Bazzite.