> Thunderbird seems to be somehow apart from Mozilla now
I don't think that's the case.
"Thunderbird is part of MZLA Technologies Corporation, a wholly owned subsidiary of Mozilla Foundation."
Thunderbirds sourcecode is literally part of the same mercury codebase as Firefox.
Thunderbird does have a very small team, and I think everyone that uses it should considering donating.
Thunderbird has always been mozilla. They split it out into the other company a few years back.
Yeah it's all a bit complex (just like the US tax code, I suppose). MZLA (which makes Thunderbird) is a subsidiary of the Mozilla Foundation. The Mozilla Corporation (which makes Firefox) is also a subsidiary of the Mozilla Foundation. In practice, this means that the people running Firefox day-to-day aren't the people running Thunderbird day-to-day, although of course they do talk, and technology choices made in Firefox can and do effect Thunderbird, just like they effect e.g. Zen Browser or Tor Browser.
(Also, someone help a non-native speaker: I think the "effect"s above should be "affect", but for some reason that looked wrong here. Why is that?)