f15-testing contains thunderbird 6. Version 1.2.1 for thunderbird-enigmail doesn't work with version 6 anymore. However I was able to install the 1.3-version as XPI provided by the enigmail-project and that seems to work fine. So it would be great to get an updated thunderbird-enigmail in f15-testing.
thunderbird-enigmail is not provided by the Fedora project itself. File a bug wherever you get the package from.