Hello, I just ran into this same problem yesterday. It's easy enough to resolve. The problem is that the updated nss*-3.12.8 version files required by the thunderbird update have not yet been pushed to the standard "updates" repo.
I fixed this for myself by browsing to a F12 "updates-testing" mirror site (for i386) and then used the wget command to download the suite of nss files. I then used "yum localupdate nss*.rpm", followed by "yum update thunderbird".
You can probably achieve the same results by doing:
yum --exclude=thunderbird update
yum --enablerepo=updates-testing update thunderbird
The nss related files on my system that needed updating were:
Nov 02 22:22:08 Updated: nss-util-3.12.8-1.fc12.i686
Nov 02 22:22:09 Updated: nss-softokn-freebl-3.12.8-1.fc12.i686
Nov 02 22:22:10 Updated: nss-softokn-3.12.8-1.fc12.i686
Nov 02 22:22:11 Updated: nss-3.12.8-2.fc12.i686
Nov 02 22:22:12 Updated: nss-sysinit-3.12.8-2.fc12.i686
---------- Post added at 02:14 PM GMT ---------- Previous post was at 02:03 PM GMT ----------
You got that yum error because you were trying to install one package at a time (I think). You would need to use the "yum localinstall nss*" command, installing all of them at once, making sure you have all the updated nns-* package files downloaded, and let yum resolve the dependency order. If you missed any, then the transaction would fail. Either of the methods I outlined should work.