calling it garbage seems kinda harsh, but I think they are moving more to using a javascript rendering method instead of xul. I remember reading about it a while back. I don't really like it either and one of the first updates from back then broke a lot of UI that had been working ok. I am not really sure what the problems are with working with xul though, but I think firefox moved off it a long time ago too. I feel like thunderbird's user base is more the type to want to use thunderbird because it runs like a local first desktop style app as an alternative to using a web interface to their email. At least that's what I like about it.
And that's the moment when firefox became garbage - just another chrome-alike, except slower and more resource-hungry. It had been getting worse for a decade prior to that, but dropping xul and breaking a ton of my extensions and customisability was the (large) straw that broke the camel's back. Sound familiar yet?
Exactly. Which is why moving their UI to a worse, javascript-powered, uncustomisable, web-alike trash UI is a bad thing. And a big part of why everything they've done in the last ~10 years has been garbage. And why I'll almost certainly be switching to something that isn't thunderbird next time I'm forced to upgrade it.(forgive my tone, nothing against you, I just get emotional when morons take an excellent piece of software I've been using for decades and turn it into broken, unusable trash)
[1] https://www.jwz.org/doc/cadt.html