try this add-on https://addons.mozilla.org/en-US/thunderbird/addon/importexporttools/

Hi Mark, Thanks for your time, much appreciated. Have downloaded ImportExport Tools3.2.1. Tells me there is a later version 3.2.4, can't get it to download. I can't find anywhere in this Add-on how to download a email file to a CD or USB flash drive. Regards Vic. Ps. Using Windows 10

Select the messages you want to save. Look for ImportExportTools in the menu; you should see an entry for exporting the selected messages. You'll be offered several formats. HTML makes for a highly accessible storage format and IIRC it can preserve attachments too. Eml format makes a verbatim copy of each message, but will need a mail program to view them. You can also use File|Save As on separate files. All of these systems fall short of regular email client message management. For searching you'll have fall back to the search tools available in your file system. And message threads aren't preserved.  :-(

VicA said

Have created numerious independant files in my Email inbox. The files are getting quite large & wish to download to a CD or USB dongle. Ant suggestions please.

Thanks for your time. Applied & WORKS. Have my downloads on a dongle. Works in Windows 10. Vic