I decided to share this tip, because it took me quite a while to figure it out, and I bet I was not the only one struggling.
I use Mac OS X (Snow Leopard) with Thunderbird (2.0.0.23) to access a mailbox via IMAP at my university (Royal Holloway, University of London).
While I am very happy with IMAP, since I love the flexibility it gives, I’ve been struggling with archiving old precious emails, since my account has a very limited quota, especially compared to the luxury of 7gb on gmail and the likes.
The solution to my dilemma was rather simple: you can create any number of folders under “Local Folders” in Thunderbird and simply drag and drop any amount of emails from your IMAP folder to the local one. That’s quite obvious, I know. However, keep in mind that if after moving emails to the local folders, the same emails still appear in the inbox of your IMAP account, for example accessing the same mailbox via webmail, you might need to compact your inbox for the actual synching to be reflected. You can compact a folder right clicking on it and choosing “Compact”.
Hope this makes sense…