FUTO cannot decide to publish on the F-Droid main repository. The F-Droid team compiles all the apps that get published there themselves, so they decide what to publish. Well, and the F-Droid team will not publish FUTO keyboard, because it’s not open-source.
Thunderbird had a redesign not too long ago. I mean, maybe you still consider it old-fashioned, but did you check you’re on the latest version?