There is an issue with Thunderbvird that is being worked on, in the meanwhile turn off email scanning in SecurityCenter > Virus and Spyware Protection > Real Time Scanning > Settings and for the DropBox problem try the following:
Click Web and Email Protection
Click Internet Connections for programs
(Hint: Click the word 'Program' at the top of that list to make it alphabetical)
Locate Dropbox and click Edit
Make sure it has full access and turn Netguard off and click Save
Other advice I've read online is that if you are in thr habit of using the Inbox to store emails, that isn't a good idea as the Inbox is constantly scanned. Make sub foilders to store emails and try to keep the Inbox empty.
That goes for any email client.