> they are moving more to using a javascript rendering method instead of xul

  > I am not really sure what the problems are with working with xul though

  > I think firefox moved off it a long time ago too

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?

  > 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.

(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