I don't know how building Thunderbird is now, but last time I tried it I came away with the distinct impression that ascending a character in Nethack is probably easier.
You know the last time I dared to even think about potentially engaging the Thunderbird team I regretted it pretty severely. The new messages badge was changed to an unread message count. The official response on the PR (that someone else had opened) was along the lines of "screw you, we know best how to use an email client" and even making it configurable was not in the cards.
In that context why on earth would I bother trying digging into documentation of questionable quality to figure out an archaic build process? Mind you in the decade or so since that issue was raised the badge has been made configurable and can show up to 99 unread or new messages. Nobody ever has more than 99, right?
Trying to dig into the theme stuff was… interesting. The documentation is exceptionally sparse, but the tooling itself was terrible. Things fail silently which means that you often have to restart to render any changes to your theme (instead of uninstalling or reloading it). Of course you'll probably also have to manually reload the theme since it won't persist across restarts if there was some sort of silent failure.
All in all interacting with Thunderbird feels more like interacting with a black box than an open source project. So no thanks, I'll pass on that kind of user hostility.
It is unfortunate, I agree. You'd think that after playing the game off and on, sometimes quite intensively, for three decades now, I'd have been able to finish it once at least.
Do they accept PRs?