I'll never know
I switched to Betterbird (at work) at the same time as I switched to LibreWolf, and for basically the same reason (Mozilla's determination to make their stuff as crap as possible).
Betterbird just looks the way a gooey email client should, imo, and I could never figure out how to get that look on Thunderbird (or rather, get it back, because that's the way it used to look).
At one point I was actually forced to go back to Claws Mail, because the real estate taken up by useless guff in Thunderbird was vast, and I could find no way of disabling it.
Finding any space for the actual message body of emails was a struggle. Meanwhile, the few bits I did manage to disable somehow obliterated the "Compose New Mail" button, which I never did figure out how to get back, and so I ended up having to use the (now hidden) menu to compose messages.
I think it had something to do with a Microsoft-esque "ribbon" thing that I would've chewed my own arm off just to get rid of. But honestly, who knows.
So now you have 16 convoluted and non-intuitive ways to do everything, except for the things you actually want to do, which have been removed entirely (or hidden away so deeply you'll never find them).
As one perfect example, I ended up having to search Reddit just to figure out why Thunderbird was double spacing every new line. Turns out there's a setting for that, the name of which escapes me (it's one of those obscure things you need to know the name of before you know what to search for), but the point is why the hell is that the default? In what alternate universe do people write like that?
Then there's this weird thing when you're setting up a Caldav account, and you go to look for the calender, and it's just not there, and the menu item for it is greyed out.
Several days of googling and redditing found nothing relevant.
Then by chance I hit alt-tab while trying again, and there it was ... some pointless confirmation window, that I was forced to "OK" before it would compete setup.
Why was this window not given focus?
The litany of Mozilla's crap design choices would fill a library.
Had I known about Betterbird, frankly I would've switched to it years ago.