Mail Exporter Pro - Easiest way* to batch/bulk export Mail.app mailboxes/folders (including subfolders) into an easy-to-import format for Thunderbird via:
ImportExportTools† "adds some tools to import and export folders and messages". Also allows exporting folder indexes to compare contents, etc: ImportExportTools > Export all messages in the folder > just index (CSV).
Another handy add-on is Theme Font & Size Changer‡ which "lets you change the global font size and font family used for the user interface". Or go manual with userChrome.css.
You can grant Thunderbird access to your Mac contacts if desired.
Remove the splash screen/pane: Thunderbird > Preferences > General > uncheck "When Thunderbird launches, show the Start Page in the message area".
Apply a custom set of columns to all folders by clicking the tiny icon at the far right of the column list and then "Apply columns to..."
* In Mail.app, you can highlight multiple mailboxes > "Export Mailbox..." > Options > Export all subfolders > Choose, but the resulting mbox files are all named "mbox".
† For Thunderbird versions above 60.*, use Christopher Leidigh's fork, ImportExportTools NG. Import options differ between right clicking "Local Folders" and its subfolders: the former offers only "Import mbox Files" (or no option for Maildir-based message stores), while the latter provides both "Import mbox Files" (or "Import Maildir Folder") and "Import EML Messages".
‡ No longer compatible as of TB 68; try this instead:
/mac | Feb 05, 2017