I've installed the enigmail extension from http://enigmail.mozdev.org/, which used to work perfectly in fc5, but now there is some sort of problem that causes thunderbird to quit/crash/segfault when I view some messages that were either signed or encrypted (even by myself). It would be nice to either figure out what causes this crash, or package enigmail in core/extras to match the official fc version of tbird.
This should probably be reported to the enigmail people... I can't possibly fix crashes caused by third party extensions, which may or may not be binary compatible with stuff we ship.
enigmail's page clearly says that if a distro packages a custom-compiled version of thunderbird, they should really also package a custom-compiled version of enigmail itself. Maybe I'll look into packaging it into Extras, though in the past, the official plugin has always worked fine.