I just switched from the regular Apple Mail app, which I had been using daily since 2008, to Thunderbird. I had been having problems with Apple Mail's program's "message tagging" feature, which I used for marking which emails I had to respond to. This feature became a daily nuisance, and after repeatedly calling Apple's tech support and jumping through all sorts of hoops, I finally decided to try Thunderbird. What a breath of fresh air! And yes, Thunderbird is Open Source software.
I've got the following Thunderbird plugins / extensions installed:
I also disabled the "Lightning" plugin because I don't want to use Thunderbird for managing my calendar. For this, I'll continue using Google Calendar (via Apple's Calendar.app).