Backing Up FirstClass Data
Click on a saved mail folder (or several at once -- I wouldn't do more than about 6 at a time.) In this screen shot, I have the folder called "VPN" selected.
Go to the File menu and select "Export."
Choose a location (e.g. your Desktop). Optionally, make a new folder, like "old mail." Click "Choose."
You'll see a small window showing the progress of the download.
When it's through, you'll have a local folder for every source folder you had selected. Inside each of those folders, you'll have a folder for every message that was in the source folder, and that folder's name will be a concatenation of the sender, subject, and date of the original message -- truncated if any of those things was too long. Inside each of those folders, you'll have the actual email message, in three formats, plus any attachments, in an "attachments" folder. It'll look something like this (in the case of my "VPN" mail folder):
Q: This seems unwieldy/awkward/stupid.
A: It is.
Q: What if I just want the messages and I don't care about the attachments?
A: Follow the same directions, except that instead of choosing "Export," choose "Download" -- the result will be a folder for every folder, with one rich text file for every message, file names based on subject lines of messages. Slightly less ridiculous but you lose the attachments.