Under what circumstances?

I wouldn't expect Thunderbird to automagically start using another mail client's data files.

Have you tried Tools|Import in Thunderbird? I don't recall if TB imports from Eudora.

And there is this: http://kb.mozillazine.org/Importing_from_Eudora_%28Thunderbird%29

my eudora is failing to connect to sympatico.ca so trying to move to Thunderbird. Email accounts moved OK and Addressbook is also there.

tried to import my eudora folders   it seems to tart OK  get blue progress for lenghth od about half inch  then it seems to hang

folder for inp is listed at bottom but contains no emails

They are pointed to by DEudora.ini as being located at D:/Eudora which is correct but Thunderbird seems to looking on C: drive (which is an SSD that is why mailboxes are on D: hard drive

Is Eudora OSE still available or is it gone too (unsupported)