The successor of FMail is FMail2. The biggest change is that FMail2 supports tabs. Supporting tabs also means you can drag a tab to a new window. At times it is handy to have more instances of Fastmail available; think of tabs for Mail, Contacts and Calendar, or an extra Compose window.
Please note that FMail is no longer actively supported.
Below links for those who still prefer to use FMail.