Can someone give me an idea of what sort of money is involved?
The fun part is that Thunderbird has FAR better potential to make a difference than Firefox. The future of Firefox is, well, meh. More small bugs fixed and maybe a recovery of the stability and low level of memory hogging it did pre release 52, a bit of WebRTC support and that's really about it.
TB, however, needs but 3 things adding that would allow it to make a rather massive impact:
1 - direct support for Exchange
2 - built-in support for calendaring
3 - better HTML/rtf rendering
(optional: 4 - better built in support for crypto such as GPG/PGP and S/MIME)
The reason for that ought to be obvious: it is the only FOSS client that has the potential to beat Outlook at its own game of locking users into MS Office and, consequently, Windows (no, I don't seriously consider KDE's Kontact a viable replacement - I think that's really beyond rescue if you want something that runs on all the major OS such Windows, Linux, MacOS and -why not- *BSD).
The problem I see is the issue of funding, but that starts with getting some idea how much is actually needed for an open project as there are various routes (some of which involve disconnecting from Mozilla completely to avoid any more grandfathering). Does anyone have an idea how much the TB project presently draws from Mozilla? I've never looked into this before, but it may be worth pursuing. I can ditch FF any day for Opera, Vivaldi and a few lesser well known browsers (nope, don't like Chrome), but TB is about the only cross platform email client.