After reading a bunch of negative comments here, let me add a little on the bright side. I've been using Thunderbird for many years, currently both at home and at work to manage gmail accounts, pop at home, imap in the office. It works great for me, with a few annoyances but nothing serious. As for the donations, Thunderbird seems to be somehow apart from Mozilla now, so I don't think much about specific org structur…
There's been a few ups and downs along the way but I've found it generally "just works" and gets out the way, which is exactly what I want in an email client.
I've tried almost every single email client I could find on Linux, and several on Windows (including Pegasus mail, if anyone remembers that), but always come back to Thunderbird.
I've been a regular donator to the project ever since they spun it out to MZLA Technologies Corporation.