merging old and new Eudora and Thunderbird files (Windows 7)

Two friends have suffered hardware failures, and in both cases have
started anew, with the same software.

Eudora user: hard drive fail; managed to restore OS etc. from image (to
new drive), and get her going again with Eudora. Will be same version
(development stopped years ago). I have subsequently managed to recover
_I think_ most of Eudora folder from the old drive.

Thunderbird user: complete computer fail (dead behaviour). I don't think
it's the HD, so anticipate being able to recover TB
folders/profiles/whatever from it (I haven't visited her yet). Have got
her going (remotely!) on replacement (W7) computer; latest TB
downloaded, but I _think_ it's not very different (I think she had it
set to autoupdate; failure was recent).

Want to restore old emails from before their failures:
o without losing any new emails (i. e. "merge" old and new)
o with access to files only - can't _run_ anything on them I think): i.
e. it _isn't_ like exporting from a running machine to a new one.

Any pointers for either software?

Thanks.
--
J. P. Gilliver. UMRA: 1960/1985 MB++G()AL-IS-Ch++(p)Ar@T+H+Sh0!:`)DNAf

"He who will not reason is a bigot;
he who cannot is a fool;
he who dares not is a slave."
- Sir William Drummond

Above all things, use your mind.
Don't be that bigot, fool, or slave.