ImportExportTools is the new name of MboxImport and gives you a host of new features for exporting and importing Thunderbird messages. Now the extension allows you to export and import messages with more flexibility, with the following options:

Tools Menu --> Import/Export in mbox/eml format
Context menu on folders panel --> Import/Export in mbox/eml format

You can exporta folder in a single file (mbox format), with even the subfolders if you want. You can also export all messages in single files (eml format) and export all messages in single files (html or plain text format), with an index.