11.3. Why is Enigmail incompatible with my Thunderbird / IceDove / ... ?

Enigmail is only compatible with official releases of Thunderbird and SeaMonkey as provided by the Mozilla Foundation and its subsidiaries.
In particular, releases shipped with Linux distributions (such as Ubuntu, Red Hat, OpenSuSE or Debian) cannot be supported. If you want to use Thunderbird / SeaMonkey / IceDove / IceApe created and installed by your distribution, then you also need an Enigmail release created by the same distribution.