@sihaha @fedora FWIW Firefox in Silverblue is native and works with flatpak'ed KeePassXC but comes without extended codecs. These codecs are in the flatpak version which doesn't work with KeePassXC, though.
In Silverblue I added some repo and installed ffmpeg with all codecs. Bazzite, though, comes with the flatpak'ed Firefox.
I hope for a solution to this soon, too. Canonical provided a flatpak portal for web extensions in 2022 or so but it's still in discussion limbo. AFAIU the relevant group aims for a proper solution and wants to make it right. I'd really like to use other flatpak'ed browsers with KeePassXC.