Antivirus program claims Thunderbird.exe has a virus
From MozillaZine Knowledge Base
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 cloud based antivirus scanner such as Panda, ESET or Kaspersky and get a second opinion. They're already uptodate and you don't need to go through a long installation process. You could also upload the file to one of the following web sites and have them check it (for free) using multiple antivirus scanners.
Malware detection has the same type of problem. HerdProtect will scan your entire PC for malware using multiple cloud based scanners.
It's not just Thunderbird that has this type of problem. For example, Nirsoft (they developed a lot of small freeware utilities) has been having problems with false positives for a long time. Things have gotten so bad that they published a anti-virus list of shame, scoring 56 different antivirus engines on how many false positives they reported for Nirsoft's programs.