Those of you using Mozilla Thunderbird in the 1.5.x branch should head on over to our Downloads section and get the latest update. Mozilla has updated Thunderbird to 1.5.0.12, including some important security updates that have been detailed on their bug list.
A better idea in the long run, of course, is to update your client to the new 2.0 branch, which you can download here. The 1.5.x branch will still receive security updates and support for a while yet, but eventually they will drop support for it along with the 1.5.x branch of Firefox.