Hi Charles,
If you can post a link to a screen recording/screen snapshots of your efforts it will help the esteemed listmate hive see exactly what you see:
-Firefox process steps to export and import the html file
-Location, size, and date of the html file on the USB drive shown in Windows Explorer or similar
-Contents of the html file shown from a browser or text editor


Any shared debugging effort will benefit from observing the steps you actually took combined with audio narration that describes the onscreen activity.  Regretfully I cannot recommend a free screen recording tool with web upload and link sharing- perhaps you already have a tool.  

Firefox has a sync service that could help as well. Sign-up for a Mozilla account and opt-in to synch on both computers and they will eventually share the same browser bookmarks. Your situation is a special case : a one time sync of the new computer from the old.
Firefox sync
https://www.firefox.com/en-US/features/sync/

Regards,
Wil Blake