From the Tools menu choose ImportExportTools > Import Mbox File. In the sheet that appears, enable the 'Select a directory when searching the mbox files to import (also in subdirectories)' option and click OK. In the resulting window select the master folder (not the .mbox) that you've exported from Mail and click Choose. You'll be asked to affirm that you want to import each mbox archive within that folder. Choose to. (You can also enable the "Don't ask me again" option and the remaining mbox files will be imported without need of your confirmation.) Each folder archive will be imported into Thunderbird and bear its original mbox name--mbox724, for example. Feel free to rename these items.
When you later wish to revisit these messages, just launch Thunderbird and choose the appropriate archive.
Sign up for Computerworld eNewsletters.