I've found Mail.app to be prefectly good as a home email client, however I would have reservations about using it to manage and answer the large amounts of email I get for work, so I can understand that you'd rather use something more streamlined. Have you tried Mulberry? Ugly interface, but if you're used to Mozilla, you probably won't mind

It is a pity that Canon didn't just go with TIFF. If you collect together all you can find out about the format and send it to the guy that writes GraphicConverter, he might just add it - he's been known to be quite helpful with such things.