Major Changes since v1.9
The Encryption and Signing buttons now work for both OpenPGP and S/MIME. Enigmail will chose between S/MIME or OpenPGP depending on whether the keys for all recipients are available for the respective standard.
Support for Pretty Easy Privacy (pEp) is implemented in Enigmail. pEp is enabled by default for new users.
Support for the Autocrypt standard. If Enigmail is used in the "classical mode" (with pEp disabled) then Autocrypt is enabled by default.
Support for Web Key Directory (WKD) is implemented. Enigmail will try to download unavailable keys during message composition from WKD. If you use GnuPG 2.2.x, and your provider supports the Web Key Service protocol, you can also use Enigmail to upload your key to WKD.
The message subject can now be encrypted and replaced with a dummy subject, following the Memory Hole standard for protected Email Headers.
The keys on the keyring are automatically refreshed from keyservers at an irregular interval.
Enigmail was converted into a "restartless" addon. That is, once you installed Enigmail 2.0, subsequent updates will be installed without needing to restart Thunderbird.
Keys are internally addressed using the fingerprint instead of the key ID.
Fixed defects
In addition to the above, we fixed quite some defects:
https://sourceforge.net/p/enigmail/bugs/search/?q=status%3Afixed+%26%26+_fixed%3A2.0.
