I'm still hanging on with Thunderbird even though they did hideous things to the UI a while back, which broke my long standing workflow in a single update.
I tried Betterbird but IIRC it didn't work properly for me, nor did it fix the UI issues that were bugging me, so for a while i pinned TB to the version before the "upgrade". But alas I eventually (I think initially by accident) relented and installed the current version with a few tweaks to help my sanity.
It still irks me that TB decided to force a major change to the UI onto established users without any choice in the matter. They should take a leaf out of Libre Office's book here, which tends to leave established layouts alone whilst offering the latest whizzy UI to new users.
I still miss the old UI but for now I'm coping with the new one. Meanwhile they're developing stuff in the background and thankfully, day to day, it rarely affects me... I wish they'd fix the HTML email editor as it's really clunky and buggy... For starters, when i begin an email in plain text (my default) and happen to add an element such as emboldened text which triggers it to convert the message to HTML, the text size instantly (and irreversibly) shrinks to a tiny font meaning I have to select all and enlarge it again.
And if you've ever tried inserting a table.... That's excruciating!
Still I'll take TB over Outlook or Gmail any day - Outlook is so shockingly bad I don't think TB could even deliberately become worse. But then again I'm often faced with businesses and users who are ditching thunderbird for Outlook (or worse, Apple Mail; or worse still, Windows Mail!) and I honestly can't get any coherent answers from them as to why.. I've been told that Thunderbird "looks too old fashioned" or that "I just prefer Outlook" even through there's never any compelling reason. Sad but true, unfortunately.
For me, nothing has ever come close to the lightning fast search in Thunderbird. Let's just say that I'm not the best at organising my email, yet the quick search in the inbox, easily expanding to a full search across folders and mailboxes means that I can always find what I need within moments. Meanwhile I can never find anything in Outlook, including recently the search bar itself which they've permanently moved into the goddamn title bar (seriously everybody, stop doing that!)..