In the earlier version of #Thunderbird with the plugin Enigmail, the keys were stored in the gpg key ring
Some specialists argue that there should be only one encryption key storage on a system and they advocate for the gpg keyring.
I can't judge whether the keys in the gpg-keyring are really more secure than in Thunderbird, but it makes sense to me that you should only have one store for encryption keys. It may be that the integration of the gpg-keyring with regard to other operating systems would have caused delays in the changeover phase.
Now the TB68->78 changeover is long over and it would be time to give Linux users full control again with all their practiced commands over their collected keys.