Speaking of Thunderbird: There's an easier way to backup

Skip the copying to local steps until you actually need to restore the mail. Just tar or zip the whole backup profile after the download has finished and keep the archive for later. Then when you need it, unzip it, replace your current backup profile directory with what you just unzipped, and open Thunderbird. Since you last closed Thunderbird in offline mode it should still be in offline mode, so Use Profile Manager to start the Backup profile in offline mode and you shouldn't have a problem viewing and copying your old emails without Thunderbird trying to sync with whatever's on the IMAP server now. Until they kill Profile Manager in Thunderbird, that is. Then you'll need the standalone Profile Manager app as well.