In hindsight, I'm surprised that a kerfuffle/fallout of this magnitude did not occur much earlier on in the life of Mozilla Thunderbird's development for signs of discontent with the way it was (and still is) being developed have been around for years. Moreover, much of that discontent could have been alleviated by Thunderbird's developers if they had made only minor changes to the program.
The old adage 'try to please all and you'll please none' is true—well, it's essentially thus—so one would expect Thunderbird's developers to have to make some hard decisions about what aspects of the program should be developed in the absence of unlimited funds.
That said, we need to stand back for a moment and look at the history of email clients over the past 20—30 years or so to examine what's gone right and wrong with their development (and especially so with Thunderbird).
Early on in the 1980s we had enthusiasts who wrote email clients and then have users pay for them (for example, Steve Dorner who developed Eudora Mail which was a well respected email client). At the time this situation was fine as both developers' enthusiasm and the fact that they were profiting from their programs meant that they would continue to support them, and competition from rivals ensured that these products would receive ongoing development.
Unfortunately, this situation didn't last, when Microsoft launched Windows 95 it bundled with it an email client for 'free' and that changed everything. Being seemingly free and conveniently bundled with the O/S—which was quickly becoming ubiquitous—meant, like the O/S, it too became the dominant email client. The fact that it was a poor client when compared with others didn't matter as much as being 'free' and convenient to use (it being always immediately to hand). And if more demanding users found its limitations were an issue then they could benefit from the bigger and better Outlook bundled within MS Office.
What was left of the earlier client market struggled on with little further development or was bought up by large companies as afterthought products to bolster their product lines only for these companies to later discard them altogether (after they'd realized there was little profit in them—as much of the sales had to be plowed back into product development). Simply, Microsoft's business practices put the kibosh on most email client development, essentially, the email client ecosystem fell apart.
(A classic instance of this was Eudora Mail which was bought by Qualcomm which it soon discarded. Eudora was an excellent client that I used for years right up to the point where its age and lack of necessary updates meant that it became untenable to continue using it.)
Many mail users were concerned about Microsoft's sudden monopoly over the email client market and that's how Thunderbird was born (how that happened is overly involved to discuss here).
What happened next was the proliferation of web mail and the likes of Google's Gmail. Now with both web mail and Microsoft's email clients being widely available the standalone POP/IMAP client market was further squeezed and marginalized, essentially this left Thunderbird and a few stragglers to eke out an existence in what was left of their sector of the market. There being now essentially no money for development, clients such as Thunderbird stagnated for years. And this is why many (including me) stuck with better albeit then-dated clients such as Eudora and we did so until the very last moment.
Eventually, with no effective competition we were forced into using Thunderbird and we didn't do so willingly. The reason why many of us found that using Thunderbird was such a problem was that things (often of a rather basic nature) that were simple to do in our old clients were either much more awkward to accomplish in Thunderbird or were not possible to do at all - except with perhaps the assistance of plugins/addons—and then there was never any guarantee that suitable ones would be available. (I can still give instances of features in Eudora that have no equivalents in Thunderbird and where there are still no plugins/addons to fill in these missing gaps.)
Nevertheless, I've installed plugins/addons into TBird to ease the pain as much as was (is) possible. What is important to stress here is that my use of plugins/addons was essentially to add necessary/utilitarian features that should have been in Thunderbird from the outset—moreover, most are simple and would have required little coding to incorporate them into the base code.
Now, to make matters even worse, Thunderbird's developers repeatedly broke plugins/addons with almost every major upgrade—and they did so with such frequency—and such predictability and alacrity—that it seemed almost a mantra for them to do so on every occasion.
(When a feature worked perfectly well until an upgrade and then afterwards ceased to do so, one's anger became very palpable. Unfortunately, this has been an all-too-frequent occurrence.)
Thunderbird's developers always had some excuse for breaking plugins/addons. Usually it was that hoary old chesnut 'security'—that forever bastion of an excuse that developers inevitably wheel out whenever they do not want to do something that users need or require. In such battles poor hapless users always come off second best.
Unfortunately, as most open source programs are free to end users, there is little incentive (and certainly no monetary incentive) for developers to do what users require. It's not only Thunderbird's development that has suffered as a consequence, there are many others; for instance, GIMP being awkwardly different to Photoshop and similarly LibreOffice from MS Office/Word. (In these cases, developers have made little attempt to ease users' transition from widely used commercial software programs to their open source counterparts. The same can also be said of Linux's developers.)
In my opinion, the issue of developers failing to fix bad or limited code because it's boring less interesting work than it is to work on fashionable and exciting aspects of programs such as new (pet) features is likely the single biggest problem with open/free software. Unlike those who pay for software in a marketplace of similar competing products and who can go elsewhere when developers fail to respond to their needs, users of free software have essentially no leverage over developers, they just have to take and accept what they're given. Moreover, I cannot foresee how this problem can be easily resolved.
I've railed for years over Thunderbird's many issues and limitations here on HN and elsewhere, as well as having provided considered feedback directly to its developers and in each instance their response has been essentially nought!
Right, they have no obligation to respond (other than perhaps the fact that they've persuaded users to use their product which takes time and effort). However, I would suggest that in a more competitive environment they would at least be compelled to provide reasons for why they can't provide a requested feature or why it was undesirable for them to do so.
Unfortunately, from my dealings with Thunderbird's and Firefox's developers, Mozilla, as an organization, has been and still is notoriously bad at explaining itself, that is why it does certain things and not others that many users would deem appropriate for action. And, in my opinion, this is very bad for the free software movement in general (Mozilla being such a big and influential player).
Clearly, I am not fully cognizant of the facts surrounding the blowup between Mozilla and Betterbird's developer but as mentioned at the outset I'm not surprised that these matters have flared into a major issue (it had to happen eventually given the litany of issues).
If my point isn't yet fully clear then I'll summarize by saying that almost every software user faces difficulties with bad, ergonomically-unsound software on a daily basis but it often becomes an intolerable problem when there is no effective competing product to which they can turn for a solution. And not having any effective competition to Thunderbird is the situation many of us users continue to face. Thus it's little wonder some of its users' tempers will flare out of sheer frustration.
Thunderbird's developers are either too self-absorbed to realize this fact or they simply do not care. Why should they when they've no effective competition?
If nothing else, Betterbird's existence may bring wider attention to Thunderbird's problems—and that's a good thing to happen.