Links from e-mail client to Firefox do not work

I recently switched to a Flatpak install of Firefox. I then realised that I cannot open links from Thunderbird embedded in an e-mail.

Clicking on the link, I get the message “Keine Anwendung verfügbar” (in English somewhat like “No app available”). I then found out that it must have to do something with the fact that Firefox is not the default browser anymore. But I cannot change this in “Settings”. I gather or maybe it has to do something with the xdg-setting, but the tip “xdg-settings check default-web-browser <your_flatpak_browser.desktop>” also did not help. What can I co to make downloads from an e-mail possible?
It is really annoying…
Thanks for any advice