I asked this before but it might have been buried. Can I run this in a web browser because when I go to the site it wants me to download.

Well, I wouldn't like AI in any communication client of mine. Perhaps if it's local to my box I would like that, but this solution really seems cloud based, meaning one could have an AI crawling over one's data, to do whatever it wants with it. And local solutions usually are not as "good" as the cloud ones for whatever reason (hardware availability, data, and so on):

for users on less powerful hardware, the development team has integrated NVIDIA’s confidential computing to keep any remote processing secure. Rest assured, those who prefer to skip AI services can continue using Thunderbird without these extras.

There's still tuta, or even /e/ (now a days murena), which still seem safer privacy wise than this new thunderbird option.

I'm really hoping for a "librewolf" kind of fork oriented to privacy, and betterbird doesn't offer anything like that. The phoenix project has a safer user config for both firefox and thunderbird, but that doesn't get rid of components (well perhaps it could possibly turn them off, though to make sure they better get ripped at build time).

Does any one know if this new TB service would offer caldav and carddav services as well? I didn't see anything on stalwart advertisement.

I already have tuta and libreoffice