BTW just FYI I have not tried this again as it was unstable even though of all these tries.
As this issue continues to resurface e.g. for other extensions too, I can only point to this here to and let you people argue for support of a proper solution:
The extension needs to use native messaging to communicate with the desktop application and due to flatpak’s sandboxing this is blocked.
Theoretically, similar workarounds as I proposed for KeePassXC could be tried.However a proper solution would (likely) involve both flatpak and Mozilla to provide a compatible way to communicate in such a case. See and support/upvote these issues:
- Mozilla Firefox’ side: https://bugzilla.mozilla.org/show_bug.cgi?id=1621763
- Flatpak’s side: NativeMessaging portal for sandboxed browsers · Issue #655 · flatpak/xdg-desktop-portal · GitHub