

It works, but due to Flatpaks being quite isolated, you need to put put a proxy in between and adjust some settings.
I believe that this is the guide I followed: https://discussion.fedoraproject.org/t/how-to-run-firefox-and-keepassxc-in-a-flatpak-and-get-the-keepassxc-browser-add-on-to-work/19452



Lucky me needs Proxmox only for self-hosting and loves it :)