It looks like the package got corrupted during the download - could you please open a terminal, type
sudo rm /var/cache/apt/archives/thunderbird_14.0+build1-0ubuntu0.12.04.1_i386.deb && sudo apt-get install thunderbird
(to remove the broken package from the cache and force apt-get to download it again) and tell me if that works?
It looks like the package got corrupted during the download - could you please open a terminal, type apt/archives/ thunderbird_ 14.0+build1- 0ubuntu0. 12.04.1_ i386.deb && sudo apt-get install thunderbird
sudo rm /var/cache/
(to remove the broken package from the cache and force apt-get to download it again) and tell me if that works?