Tuna's Command-Line Migration Adventure Episode 3: E-Mail!

[Tuna]

I want to switch to mutt. I really do. However, I am currently using 
Thunderbird, which I configured to download messages and delete them 
from the POP server. So it would be kind of inconvenient for me right 
now, as I don't want to lose the over 9000 messages I have in my 
Thunderbird inbox.

Is there a way I could back up all these messages in a mutt-compatible 
format? Or maybe I could use IMAP to... do something?

Bonus round: I have a very well-calibrated spam filter on Thunderbird, 
is there a way to back that up as well and use it with mutt?