You can't email folders. A folder is technically just a collection. For a regular folder in your filesystem, I'd say you should zip it up and send the zip file. You can drag and drop individual messages onto a new message to add them as attachments. Drop them onto the addressing box at the top. Another approach would be to export the messages as .eml files, zip up the eml files and attach that zip file to your new message. This add-on: https://addons.mozilla.org/en-US/thunderbird/addon/importexporttools/ is useful for exporting multiple files.