Virus in Portable Firefox?

Hello,

I just found a Trojan in my PortableFirefox.exe. My Scanner (Antivir) told me that it found a trojan called: TR/Dldr.Zlob.VV.2. When I tried to download a fresh one from different mirrors, it always found the trojan in the "fresh" archive again. So I am not sure if the PortableFirefox.exe is reinfacted everytime again or there is a trojan on the mirrors???

N.M.