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?)
(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?)