RPM of the new version of messaging client is available in remi for fedora 13, 14 and enterprise linux 6 (RHEL, CentOS, ...). This is a backport of F-16 version.
Start by reading :
Thunderbird Release Notes
As always:
yum --enablerepo=remi update thunderbird\*
enigmail (version 1.3.5) extension are also available.
lightning is not yet available because of issue with some incomplete langpacks.
RPM of version 10.0 are availalble for fedora 13 and 14, and for enterprise linux 6...