I’m not sure if it’s fair to describe a "wholly owned subsidiary" as unrelated.

> Thunderbolt is funded through a dedicated investment from Mozilla and is being developed by a separate team focused on enterprise AI products, distinct from Thunderbird’s donation-supported consumer product work.

Emphasis mine.

Yes, it's unrelated. Each one has its own resources and roadmaps. They are totally not dependent on each other. Thunderbird and its roadmaps/projects are not affected by Firefox' earnings at all. One's development doesn't affect the others.