why are the repos so out of date
I've recently had to manually update Firefox because the version on the Fedora repos is way out of date. Thunderbird is now on 37, the repo has 31. Are they never updated?
What version of Fedora are you?
The newest versions as of right now are firefox-42.0-2.fc23.x86_64 and thunderbird-38.3.0-1.fc23.x86_64. So what do you get when you do
dnf upgrade
? Fedora 19 and 20 do have Thunderbird 31. Please consider upgrade to Fedora 23.