Mozilla Thunderbird is no longer actively developed by the Mozilla Corporation.
This does not make it a bad email client, it just means that Mozilla doesn't think that it can effect the future of the web with Thunderbird anymore.
However, I think they still can.
They would have to do a lot of work to integrate GPG into Thunderbird, thus enabling people to easily encrypt/decrypt emails if needed (and make it easy to do so).
I personally use Thunderbird with Enigmail for a good portion of my communication. Luckily, since Thunderbird is licensed under the MPL, it is possible for someone to take the Enigmail code, the GnuPG code, and the Thunderbird and integrate it into one large codebase where everything is licensed under the GPL. Unfortunately, not many fixes would be able to make it upstream.
In any case, I personally use Thunderbird instead of the Gmail client because I can a) download my email and b) encrypt/sign emails to friends (even if I don't care if the government sees any one particular email) and c) for the peace of mind.