If you’re a Eudora user cast into the world of Thunderbird you may wish to try the Penelope extension available from the Penelope page (eventually a Penelope build of Thunderbird will be available). Sadly it gives the Eudora Toolbar (which I never use and can’t seem to work out how to turn off), but it does give you the option key-click to gather messages by a particular column, which is probably my favourite Eudora feature ever.