(Context: I'm a submodule owner on Thunderbird, but haven't had much time to work on it lately. It should go without saying, but the comments below are my own view on the matter and shouldn't be considered authoritative.)
tl;dr: I think Mozilla has actually treated Thunderbird fairly well (but not quite as well as I'd like).
Thunderbird has been difficult for Mozilla to support for a while. In many ways, Thunderbird is only in as good a spot as it is because of people at Mozilla who still care about Thunderbird. There are lots of Gecko features that are used by Firefox and Thunderbird, and often, Gecko devs who make breaking changes to something Thunderbird uses will - if not fix the bug - at least alert the Thunderbird team of the breakage and point them in the right direction to fix it. They're also willing to support (some) stuff that only benefits Thunderbird, especially in the text editor. This can be hard for both teams though, like when Mozilla made huge changes to the Firefox build system; since Thunderbird imports the entire Firefox source tree, Thunderbird had to keep up or be unable to build.
As you can imagine, these difficulties will only be magnified by the (slow) move away from XUL. Worse, Thunderbird's codebase is pretty crufty and full of decisions that last seemed good over a decade ago (e.g. libmime's decision to create its own C-based object system instead of using C++ like the rest of Gecko[1]). It would take a lot of work to make the codebase not suck, and I'm not sure Thunderbird has ever had enough paid staff to do that in a reasonable timeframe. These architectural issues are the reason that message tabs in Thunderbird aren't actually multiplexed: every time you open a message tab, it actually just rearranges the elements from the single instance of the 3-pane layout to make it look right. (That's why scroll position in message tabs isn't preserved when you switch tabs.)
Thunderbird has also has significant difficulties finding a way to make money. Those who remember the Mozilla Messaging days (and the subsequent merge back into MoCo) might recall how that's when the team pushed for features like "Get a New Mail Account" (which has partnered mail providers who could sell users a fancy email address), "FileLink" (which has partners providing file hosting), and even a planned effort to do like Firefox and make money from search referrals (this fell through for some fairly strange partnership issues that I'm not sure I can talk about).
However, despite their general desire to discontinue financial support for Thunderbird, Mozilla (the Foundation, I believe) has opened up donations specifically for Thunderbird[2] which, as I understand it, gave the Thunderbird Council the ability to hire the first paid engineer on Thunderbird in several years[3]. In addition, Mozilla has worked to help Thunderbird find a new long-term home and has provided infrastructure support when they really don't need to do either.
While I think Mozilla should place a greater focus on email, I'm not sure Thunderbird is the best way to go about that, especially as Mozilla moves away from XUL. With the rise of Electron/Positron, something like glodastrophe[4] (based on the ill-fated Firefox OS email app) might have a better long-term outlook. However, trying to replicate all (or even most) of Thunderbird's features in a new application would be a huge effort and it's unclear whether the app would be popular enough to make all that effort worthwhile; I expect Mozilla would want to feel confident that they could use such an app to influence email/messaging standards.
I don't know what's going to happen in the long run, but as a Mozilla employee who started out working on Thunderbird, I still have a special place in my heart for it. I hope that one day Mozilla will be willing to revisit email, but it might take a while.
tl;dr: I think Mozilla has actually treated Thunderbird fairly well (but not quite as well as I'd like).
Thunderbird has been difficult for Mozilla to support for a while. In many ways, Thunderbird is only in as good a spot as it is because of people at Mozilla who still care about Thunderbird. There are lots of Gecko features that are used by Firefox and Thunderbird, and often, Gecko devs who make breaking changes to something Thunderbird uses will - if not fix the bug - at least alert the Thunderbird team of the breakage and point them in the right direction to fix it. They're also willing to support (some) stuff that only benefits Thunderbird, especially in the text editor. This can be hard for both teams though, like when Mozilla made huge changes to the Firefox build system; since Thunderbird imports the entire Firefox source tree, Thunderbird had to keep up or be unable to build.
As you can imagine, these difficulties will only be magnified by the (slow) move away from XUL. Worse, Thunderbird's codebase is pretty crufty and full of decisions that last seemed good over a decade ago (e.g. libmime's decision to create its own C-based object system instead of using C++ like the rest of Gecko[1]). It would take a lot of work to make the codebase not suck, and I'm not sure Thunderbird has ever had enough paid staff to do that in a reasonable timeframe. These architectural issues are the reason that message tabs in Thunderbird aren't actually multiplexed: every time you open a message tab, it actually just rearranges the elements from the single instance of the 3-pane layout to make it look right. (That's why scroll position in message tabs isn't preserved when you switch tabs.)
Thunderbird has also has significant difficulties finding a way to make money. Those who remember the Mozilla Messaging days (and the subsequent merge back into MoCo) might recall how that's when the team pushed for features like "Get a New Mail Account" (which has partnered mail providers who could sell users a fancy email address), "FileLink" (which has partners providing file hosting), and even a planned effort to do like Firefox and make money from search referrals (this fell through for some fairly strange partnership issues that I'm not sure I can talk about).
However, despite their general desire to discontinue financial support for Thunderbird, Mozilla (the Foundation, I believe) has opened up donations specifically for Thunderbird[2] which, as I understand it, gave the Thunderbird Council the ability to hire the first paid engineer on Thunderbird in several years[3]. In addition, Mozilla has worked to help Thunderbird find a new long-term home and has provided infrastructure support when they really don't need to do either.
While I think Mozilla should place a greater focus on email, I'm not sure Thunderbird is the best way to go about that, especially as Mozilla moves away from XUL. With the rise of Electron/Positron, something like glodastrophe[4] (based on the ill-fated Firefox OS email app) might have a better long-term outlook. However, trying to replicate all (or even most) of Thunderbird's features in a new application would be a huge effort and it's unclear whether the app would be popular enough to make all that effort worthwhile; I expect Mozilla would want to feel confident that they could use such an app to influence email/messaging standards.
I don't know what's going to happen in the long run, but as a Mozilla employee who started out working on Thunderbird, I still have a special place in my heart for it. I hope that one day Mozilla will be willing to revisit email, but it might take a while.
[1] https://dxr.mozilla.org/comm-central/source/mailnews/mime/sr... [2] https://donate.mozilla.org/en-US/thunderbird/ [3] https://mail.mozilla.org/pipermail/tb-planning/2016-December... [4] https://github.com/asutherland/glodastrophe