Perhaps a Good thing?
Seeing as how Mozilla have been ruining Firefox in a misguided attempt to make it into a (poor) clone of Chrome, I think having Thunderbird detached from them is a good thing.
Quite frankly, I have been moving away from Firefox due to their messing with it (if I wanted a browser like Chrome I would just use Chrome, FFS) and Thunderbird is still my go-to email client (even for webmail systems like gmail).
All I would ask, is for some decent native CalDAV implementation. The Calendar plugins always seem a bit "tacked on" and not fully integrated, and sometimes will cock up.
Also, make the "smart search" work. It is completely useless, finding either no emails, or hundreds of emails, none of them related to my search terms. The "email filter" search that was the original method is far more intuitive and works better, but you have to enable it specifically, and it only works on a "per folder" basis.
Apart from that, Thunderbird is a solid piece of software, doing what it was designed to do, and doing it well. Please don't chase stupid "UI Fashion" and other buzzword crap like Mozilla has done with Firefox, just concentrate on bugfixes and the odd feature request, and you will do well.