I would have to ask, why bother? Any attachment stored in Thunderbird is stored as plain text, not in a form where the included malware could activate. So in storage they are completely inert. Should you choose to open the attachment, or save it, your antivirus product has already shown it can detect the virus/malware. So it will stop you or anyone else doing so at the time you interact with it. The other side of that coin is you might want to try actually saving one of those attachments and see if bitdefender still thinks it is malware. False positives are common where some "malware" is concerned and suddenly finding thousands of preexisting malware infections in data that has been in storage on your system for more than 2 years sounds like a false positive really. A google search for Trojan.GenericKD lists a lot of false positives. The [https://malwaretips.com/blogs/trojan-generickd-removal/ malware tips web] site shows the threat as Trojan.GenericKD is a heuristic detection designed to generically detect a Trojan Horse. Unfortunately heuristic detection are frequently false positives. [https://www.f-secure.com/v-descs/trojan_w32_generic.shtml F-secure] does offer that this is "A generic detection has identified a program or file that has code or behavior similar to trojans." I would suggest trying one of the "other" online virus scanners and see if you get the same result.