I talked to the Debian maintainer as well and he isn't sure how it would have ever worked to automatically open Thunderbird. Also from what I can see I am not sure if using xdg-email would work for Thunderbird since they apparently do weird things for attachments, from what I have read online. I will try to setup a test environment to make sure that switching over to xdg-email will work, but as a work around until then just put 'thunderbird' in that dialog as you have already.
I talked to the Debian maintainer as well and he isn't sure how it would have ever worked to automatically open Thunderbird. Also from what I can see I am not sure if using xdg-email would work for Thunderbird since they apparently do weird things for attachments, from what I have read online. I will try to setup a test environment to make sure that switching over to xdg-email will work, but as a work around until then just put 'thunderbird' in that dialog as you have already.
Thanks,
Chris