Comment 6 for bug 1960421

I have tested adding a custom protocol handler in Firefox by creating the following entries in about:config :

  network.protocol-handler.expose.webcal
  network.protocol-handler.external.webcal
  network.protocol-handler.warn-external.webcal

and I did manage to get a new entry for the webcal protocol, but associating it with an external application (either thunderbird or xdg-open) doesn't work in the firefox snap, because the file dialog that is presented to the user to select an application uses the xdg desktop portal which remaps the path to e.g. /run/user/1000/doc/9a5d9a5e/xdg-open.

This needs testing in the firefox deb, with snap confinement out of the way.