Thunderbird mail loss, exclusion resetting after each reboot

Hello,

There's a case of the latest Norton Security on Mackbook Pro (Sierra) where Thunderbird mail was completely lost (inbox file) because of a single email containing some threat. I found more information about this where it was said that Norton cannot distinguish between single emails in Thunderbird, but treats the whole folder file as one email - very unfortunate.

Anyhow, I attempted to exclude Thunderbird folders from automated scans, just to discover the exclusion settings are gone after each reboot!

Is Norton personnel aware of this problem? Is there a way of resolving it?

Thank you