Will there be a mail-saving function or addon available for 78.2.2 (prior ImporExportTool NG) soon?

You can save one or more messages without using any add-on. Simply right-click the message(s) and press Ctrl+S on your keyboard, or use the '''File > Save As > File''' menu option. You get to choose between eml, html or text file formats. A beta version of ImportExportTools NG is also available here https://github.com/thundernest/import-export-tools-ng/issues/143 and works with Tbird 78. The author is continuing work on it, so we should be having a stable release soon.