Several bug reports have been raised on this issue, but none have a clear-cut date. Since the latest Thunderbird will be included in Ubuntu 12.04 LTS, due out on 26th April 2012, Canonical may not be allocating a high priority to updating the repositories for 10.04 LTS.
Numerous sources, including the Mozilla and Ubuntu forums, recommend adding the Mozilla-supported PPA to your software sources list. In a Terminal window:
Code:
sudo add-apt-repository ppa:mozillateam/thunderbird-stable
sudo apt-get update
sudo apt-get upgrade
Replace 'thunderbird' with 'firefox' and repeat and Firefox will also be updated and kept up to date.