There's a helpful article here: http://kb.mozillazine.org/Importing_from_Eudora_%28Thunderbird%29 You'll see that it recommends using an old version of Thunderbird; also that migrating the address book to Thunderbird is more straightforward because Eudora can export contacts data in standard .csv format. Looking on the internet there's at least one commercial program that promises to convert Eudora mail to other formats, but I've no idea if it works reliably.