A look at Thunderbird 3

Yes, it's not intuitive. But my comment was only to correct the statement that thunderbird cannot deal with the mbox format - which is how I interpreted your text. Sorry if I read it wrongly.

PS. My guess why thunderbird doesn't use ~/mail by default for local folders is that historically there have been several mail folder formats and not all of them it recognizes. However, mbox specifically is o.k.