Try this: go in Tools --> Advanced --> Editor settings, find these two preferences
mail.imap.fetch_by_chunks
mail.server.default.fetch_by_chunks
then set them to "false" instead "true" (right-click --> Toggle).
In newer versions, in this example i'm using Thunderbird 16. It's located in a different place:-
1.) Tools (click)
2.) Options (click)
3.) Advanced Tab (click)
4.) Config Editor (click, down bottom)
5.) Agree to Voiding Warranty (click I'll be careful i promise)
6.) Type "chunks" in the search box.
7.) Double Click the row and it will change to FALSE.
Have fun!
