Following my previous email, Fedora is now building and testing Thunderbird 78.3.1 for rawhide (https://src.fedoraproject.org/rpms/thunderbird). They will certainly upgrade current stable versions soon. We have made a specific documentation for Thunderbird 78 and higher for configuring Split GPG without Enigmail. Please have a look here: https://www.qubes-os.org/doc/split-gpg/#thunderbird-78-and-higher. You can already try to configure and test yourself by using builds I've made for Fedora 31 and 32 on my COPR repository.