RPM of the new version of messaging client is available in remi for fedora ≤ 12 (backport of F-14 version).
Start by reading :
Thunderbird Release Notes
As always:
yum --enablerepo=remi update thunderbird
enigmail (version 1.1.2) and lightning (version 1.0b3pre) extensions are also available.
RPM are availalble for fedora 10 to 12... thunderbird-3.1.6