Is Mozilla that involved in Thunderbird? IIRC their revitalization happened more under The Thunderbird Foundation after Mozilla put them out to pasture to die after years of neglect.
Last time I talked about Thunderbird here on Lemmy (and was downvoted because, allegedly, Thunderbird and K-9 are the exact same app, according to android@lemmy.world), I seem to recall it was however mentioned one of the differences between the two is that Thunderbird was going to include setup for Google play subscriptions (whatever that is)...
The devs description states that it's intentionally minimalistic visually and focuses on advanced features. FairEmail is way overkill for someone with a single gmail account for example.
At the time that I found it, FairEmail was the only client that met all of my needs. Like managing multiple accounts, each with multiple folders and none of that unified nonsense. It's also available on F-Droid and GitHub.