1) I installed this add-on for Thunderbird... thanks, Kaosmos! (Click the download link at the bottom of the page, then in Thunderbird you'll have to click "Tools -> Add-ons -> Install" and then navigate to the .xpi file... on mine it landed on the desktop).
2) My Kmail folders were maildir format, so within Kmail I had to do this for each folder:
- For a folder named Family, I created a new folder named FamilyM or FamilyMbox or whatever. Make sure when you are creating the folder that you select the mbox format!
- move all the mail from my original folder to the new one.
- delete the old empty folder, just to avoid confusion
4) I created a folder on my desktop called "TempMail."
5) Using the Konqueror file manager to navigate to /home/les/.kde/share/apps/kmail/mail, I copied all the ".Whatever.directory" folders into the new TempMail directory on the desktop.
6) In the new TempMail directory, I deleted the "." from the beginning of each of these folder names, so they would be visible to the importer script.
7) In Thunderbird, I created a new folder named Family (or Whatever) by right clicking on "Local Folders" in the folder tree, and selecting "New Folder"
8) Then I right-clicked on the new folder just created, selected "Import/Export," and "import mbox file," then "Select a directory where searching the mbox files to import." (Italian English?)
9) A box opened where I had to navigate to the TempMail folder on my desktop, and select one of the "Whatever.directory" files therein, and click "Open."
10) I then was asked "Do you want to import the file /home/les/Desktop/TempMail/Whatever?
I clicked "Yes!" and guess what? It worked!
11) Finally it was time to rename all the new Thunderbird folders, because the importer script adds a random 3 digit number to the folder name, to avoid the possibility of overwriting an existing mail folder.
Whew! I know this is going to be automated someday! I hope this helps someone. Let me know if it needs clarification.