Fedora ≥ 11 
yum --enablerepo=updates-testing                 update   thunderbird
yum --enablerepo=rpmfusion-free-updates-testing  install  thunderbird-enigmail

Fedora ≤ 10

yum --enablerepo=remi  update   thunderbird
yum --enablerepo=remi  install  thunderbird-enigmail

Or download from the repository

This RPM will also be available in RPM Fusion for Fedora 11 and 12 when Thunderbird 3.0rc1 will hit the Fedora repository.