Antivirus program claims Thunderbird.exe has a virus
Occasionally somebody posts a thread in the forums stating that McAfee or some other antivirus program claims that Thunderbird.exe contains a virus. Antivirus programs aren't 100% reliable, sometimes they detect a virus when there isn't one. Thats called a false positive.
Its probably a false positive if you used a Mozilla download site since if this actually occurred it'd be in the news. However, you should take it seriously. Update your antivirus program and its database and see if you get the same results. If you do, scan the file with a different anti-virus program and get a second opinion. Its probably easiest to do this using one of the following free online (browser based) scanners. None of them require Internet Explorer or use ActiveX controls.
You could also upload the file to one of the following web sites and have them check it (for free) using multiple antivirus scanners.