With the Penelope project, Mozilla developers are looking to blend Eudora and Thunderbird into one. Set to be an extension that can be installed on either mail client, Penelope is currently available in a Eudora installer provided on the Mozilla wiki. This expected release is obviously aimed principally at Eudora users, as it keeps many of the significant features of the classic mail client like the icons and layout. Many things you could do in Eudora are also possible in Penelope, like the Group Select feature and the original IMAP auto-expunge behaviour. Penelope can also import Eudora address books, filters, mailboxes and messages to make the transition smoother. Expert users will be able to choose between Eudora or Thunderbird keyboard shortcuts.
Having tested it out both on Mac and PC, I’ve found that Penelope works fine. In all, both the mail and feed reading experience feel extremely similar to that on Thunderbird, which does make me wonder if it was really necessary for Mozilla developers to work on Penelope. The only real reason I can come up with is that there still are hardcore Eudora users out there, and this is the best way to get them slowly accustomed to Thunderbird, before altogether ditching Eudora. I could be wrong, so the best would be for you to decide for yourself. Check out Penelope on Mac or PC and let us know.