As more and more people move to using Android and ios as their primary computing environment, the existence of accessible FOSS/Libre tools such as Thunderbird becomes more important than ever.
So, if they're interested or dedicated to building open tools for people to use, they can choose where to go, whether you like it or not. :)
e: By the way, have you donated to Thunderbird? If you are able to pay for the software you use, especially when it seeks to be a force for good, then you should. Even $20 a year goes a long way for an organization like this.
I don't know if there are any other public surfaces the Thunderbird team uses to figure out what to work on (outside of bug reports/telemetry and Mozilla Connect).
also an nvda user/donor
As for FX - i do use it on macos and it works perfect for me.