Thunderbird was always the lesser entity within the Mozilla infrastructure, and there have long been influential engineers within the core Gecko development cohort who did not think that Gecko engineers should be attempting to do anything to support Thunderbird. (Of course, there were other engineers who would happily write the patches themselves to fix Thunderbird if informed they broke it, so this is by no means a universal opinion within Mozilla).

In and around 2012, Mozilla was deeply terrified of its sliding market share and felt that the best way forward was to move into the mobile market. The initial XUL-based Firefox for Android proved to be a veritable resource hog, which made Mozilla push a lot harder and faster both in ending its support for XUL, and in revamping its browser engine to work a lot better at mobile devices. One of the results of this effort was the attempt to make a mobile OS, running on the most pitiful smartphones, that would use Gecko as its main application environment. Thunderbird, being a large, featureful, monolithic application with a 15-year-old codebase [1], was (and still is) not really suitable for use on a mobile operating system, and it's probably easier to rewrite the code from scratch than to attempt to modify it for use on mobile devices.

The then-CEO (his name escapes me at the moment) of Mozilla in 2011 was a big advocate of the push to mobile, and continuing support for Thunderbird was the sort of distraction that looks like a big cost that it couldn't afford in a time of crisis. So it's not surprising that they decided to stop paid development, although the announcement came as a shock to the people working on it: one person had been hired with the expectation that he would soon be leading a time of people, only to find that he would become the sole paid engineer on the codebase.

Originally, when Mozilla made the announcement, they promised that there would be a skeleton crew that would do minimal maintenance of the project. A developer, release engineer, QA person, and a build engineer if I recall all the roles correctly. Over the next few years, the people who were still doing paid development were essentially told to stop working on Thunderbird stuff, so that even the skeleton crew promised proved not to last long. In the meantime, though, several community members (myself included) were providing most of the legwork to actually maintain the project.

One of the other things that Mozilla promised was that money donated specifically for Thunderbird would remain in a pool that could only be used for Thunderbird. It took a few years for the community (many thanks especially to rkent's efforts in actually getting this to happen) to actually get access to these finances, but by around 2017, we were able to finally start getting contracts for people to actually work on Thunderbird on a paid basis. There's actually no contribution from Mozilla, except for the free hosting it provides as well as the time that core Gecko developers are willing to spend on contributing to Thunderbird.

[1] In terms of oldest Mozilla code still in active use, Thunderbird has some contenders. There are several pieces that are essentially unchanged from the public CVS 1.1 revision in 1999, and some code I found that is substantially similar to the Netscape Classic code once on MXR--I don't know if that was the 4.x version or the abandoned 5.x version. Some comments in libmime have a date in 1997.