I downloaded Thunderbird 5.0 Beta 1 at Mozilla.
Thunderbird 5.0 Beta 1 Download
and saved it to my Desktop.
I am using Avast AIS 6.0.1125 and SAS 5.0.1088 with Core: 7205 Trace: 5017 which I believe are the latest.
I scanned the file with AIS and SAS and it came up clean
I then scanned it over at VirusTotal
and it came up clean.
I then ran the saved downloaded Thunderbird 5.0 Beta 1 installer.
SAS immediately popped up and said "Trojan". The install was stopped and the file to moved to Quarantine.
Sorry but I deleted the file in Quarantine so I don't have the name of the virus.
I uploaded the Thunderbird 5.0 Beta 1 installer file using the SAS Sample Submission Tool.