I'm now a Thunderbird user, after trying about 5 times to make it my default. I'm using it with 2 email accounts with a Google Chat plugin. Email and Google Chat let me make this my default email.
There is one settings that I didn't know about that made me give up on Thunderbird a few times before. That's having the http and https links open up in Thunderbird by default. You should change the default because as great as Thunderbird is, have the web links that open up in Thunderbird doesn't work. Open up a Google Meet link from Google Chat or from an email link simple fails. Worse than that, you lose your chat window.
I use Bitwarden and when a site opens that I need to log into, I hit a roadblock again. This would screw me up several times a day.
I just realized today that I could change the default in the configuration editor. Now everything works great.
You should really consider making external browsers the default for http and https links. It more disruptive to everyone's workflow than you may realize. Just my thoughts.
