yum --enablerepo=remi-test install thunderbird3-enigmail

Or download from the repository

Warning : in this version, some translation are not completely done and the extension is unusable : this is unfortunately true for the french... (but that also one of the pleasures to work with development version)

A temporary workaround :

LANG=C thunderbird3

Stay up, on this thread, for update which should solves this issue.