I used Thunderbird daily for the better part of a decade until about 3 months ago. It's nothing personal against the project, which is currently the best serious desktop mail client available afaik. It's simply that Mozilla doesn't care about it anymore since everyone uses webmail now. Thunderbird is doing feature releases about once a year now [0], and Mozilla has pulled most (all?) of its full-time developers [1]. A few months ago, Mitchell Baker sent a message that said they were looking to detach Mozilla from the project completely, partially because it didn't have enough industry-wide impact to be worth Mozilla's resources [2].
The sad fact is that desktop mail clients are going away, and even the big projects like Thunderbird are barely limping along, which I guess is slightly nicer terminology than "pretty much dead"?
[0] https://www.mozilla.org/en-US/thunderbird/releases/
[1] https://blog.lizardwrangler.com/2012/07/06/thunderbird-stabi...
[2] http://techcrunch.com/2015/11/30/thunderbird-flies-away-from...