Thunderbird's latest version has a problem. Turn off Email attachment scanning in SecurityCenter > Virus and Spyware Protection > Real Time Scanning > Settings
You are still protected if you save said attachments to the hard drive, they will then be scanned.
This thread has some insight on that - https://community.mcafee.com/thread/50903?start=0&tstart=0
I've been having the same problem for about 2 weeks now. I thought it was TB but now realize it is McAfee. I tried to change the settings to exclude my emails, but the McAfee view page wouldn't allow me to click on "apply".....so the problem is still there. I'm reading that this issue had gone on for quite a few years now...at least since 2004.