I erased a folder from Thunderbird. How can I restore it?

Somehow -- I don't know exactly how -- I deleted a folder from Thunderbird. When I search messages, individual messages from that folder show up. But the folder itself -- which was a large one with my "general email" in it -- is gone. How can I find and then restore this folder? I am on a Mac and have Time Machine. I can locate Thunderbird, but can't go in to it to find the particular folder. If I restore Thunderbird from a month ago, I will lose all the emails of that month. I don't want to do that. I want only the folder named "letters specific".