Re: She owes me a new BS-o-meter
Here's my 2 cents, as someone who's NOT a native English speaker and has to deal with both Firefox and, to a lesser degree, Thunderbird related development: generally speaking, Mozilla lacks any sense of common sense (https://www.psychologytoday.com/blog/the-power-prime/201107/common-sense-is-neither-common-nor-sense) and direction and it's perfectly able to take all kinds of silly decisions just because. In this case however, there are a lot of past silly decisions that kind of forced Mozilla to get to this decision.
I guess that's it's much easier for them to change plans than execute well on those they have; that's how they ended up with three (soon* to be four**) different extension models - all poorly documented, buggy and incomplete. The stories of e10s (FF multiprocess) and extensions signing are fun on their own and prime examples of poorly though-out plans that did more harm than good with the community.
I think that the (serious) trouble with Thunderbird started with e10s, the new "Chrome" like extension model and the decision to remove support for XPCOM. None of these things went well with the Thunderbird and Thunderbird extension developers and many were/are thinking about forking Gecko just so they can keep Thunderbird alive with XPCOM and the current extension models.
Obviously now, Mozilla doesn't feel like maintaining a Gecko branch just for Thunderbird and, most likely, lacks the manpower to quickly bring Thunderbird up to date with the soon* to be released new Gecko changes; the Netscape Communicator 4 part is a "bit" off: Thunderbird wants/needs to stick to Netscape 4 technology while FF wants to move in another direction.
*soon - in Mozilla's world, soon may mean many things but it's never less than a couple of years.
** Chrome alike extensions