mail import from Mozilla 1.4 to Mozilla Thunderbird 2.0

I'm trying to import mail from Mozilla 1.4 into Mozilla Thunderbird 2.0. This involves moving approximately 10000 mail messages sorted into approximately 100 folders from one machine to another. Given the number of mail messages and the effort spent in sorting and classifying them, no solution that involves re-mailing them to myself will work easily.

Here's what I've done:
 1) On the old machine, tarred up the contents of ~/.mozilla/default/ . All the mail is in here.
 2) FTPed the tar file to the new machine.
 3) Untarred into ~/.mozilla/default on the new machine.
 4) When I start Thunderbird, it does not offer to import mail. It does go through the wizard setting up a new account. I am successfully retrieving mail from my POP account on the new machine.
 5) It appears that Moz Thunderbird 2.0 stores its mail under ~/.mozilla-thunderbird/RANDOMUID.default . The two mail collections are separate right now.
 6) Pulling down Tools -> Import... it appears that older versions of Mozilla aren't even listed.

So, questions:
 1) Maybe this isn't an "import" at all? If so, what operation is it?
 2) How can I point the new mail client at this big heap of old mail and absorb it, preserving folder structure etc.

(In case it matters: Ubuntu Gutsy, x86_64.)

Thanks for all relevant assistance,
 John Aspinall