It does work via Flatpak, you have to give Freetube the “D-Bus session bus” permission and then put the following in your external player settings (this launches the flatpak mpv):
External player
mpv
Custom External Player Executable:
flatpak-spawn
Custom External Player Arguments:
–host flatpak run io.mpv.Mpv <any mpv arguments you need>
What’s wrong with Thunderbird/Betterbird?