[GTALUG] Importing mbox into Thunderbird?

I'm beginning to like Thunderbird.  It replaces fetchmail, procmail, sendmail, mutt, and vim for mailing.  I miss vim, though. Now, the question on hand... I have lots of "mbox" format files of old emails. How do I import into Thunderbird?  Online talks about "Import MBOX file" under "Tools".  But, mine (68.12.0) don't have such entry.