Thunderbird actually had a big resurgence a little while back, I use it as my mobile client 🤷‍♂️ If I understand correctly it's not actually a directly Mozilla project anymore.

Personally I'm less bothered by the terms of use changes specifically than the bigger picture of mozilla consistently making choices that confuse or raise eyebrows with their core audience, letting their browser languish from a technical standpoint, and making confusing business choices that don't seem to help their financial future at all while paying executives huge salaries