The reason for the lack of implementation has very little to do with a fault on the side of Thunderbird. It's been blocked on Firefox people accepting the changes in https://bugzilla.mozilla.org/show_bug.cgi?id=441414 (which they finally categorically refused to do a few years ago).
Yeah, it's the usual open-source ego-driven squabbling.
Whatever the reason may be, users were left without this extremely useful and desirable feature for fourteen YEARS. Thunderbird at its core today acts and performs essentially identically to 2003. I've been using it myself since well before that. Development on the mail client hasn't meaningfully progressed since, well, _ever_.
