I downloaded and installed Mozilla Thunderbird in the past and tried it out. It's a free, user friendly and highly tweakable Email client (replacement for Outlook Express for most of us). I personally replaced Microsoft Outlook with it at home.
This is a very good product and will only get better. The spam filtering is excellent, it learns very quickly and hotkeys make marking (and unmarking) spam easy. No note taking functionality is built-in, however many great note taking extensions exist. If you love FireFox, you will love Thunderbird. The product is not fully mature yet, but is a good product that will continue to get better.