When you use the "Export"/"Import" method, the tags are not included. Try using the "Backup"/"Restore" method instead.

Firefox's bookmark backup format (a .json file), contains comprehensive data on your bookmarks.

In other to restore a bookmark backup file, you need to copy it from your old computer to your new one. You either can use the last automatic backup file, or perhaps it's best to create a new one in an easily accessible location rather than digging around in your application settings.

Try the steps in this article: Restore bookmarks from backup or move them to another computer. Does that work?

You can also consider to copy the places.sqlite file to move over both history and bookmarks.
Both restoring a JSON backup and copying places.sqlite will replace (overwrite) existing bookmarks, so you need to export existing bookmarks to an HTML file and import this backup afterward to prevent losing those.

Both suggestions worked. Thanks. As an aside, though the FAQ about copying bookmarks is posted, the impact of losing your tags in this situation is not spelled out. You would have to know or anticipate this bug beforehand to even search for an answer (as I did) to avoid this. This is counterintuitive. This should be posted in BOLD advising specifically that "tags" will not automatically be attached to an imported bookmark.html file when imported into Firefox on a new computer.

Hi rjgx, thank you for your comments. Most of the articles address "how to" without telling you which is the best procedure. Maybe there needs to be a master article for "Copying your Firefox stuff to a new computer" that provides best practices? In the short run, since the knowledge base is a collaborative project, you can go into the article and mark in some edits. I don't think it's possible to break the article because all suggestions are reviewed, so why not give it a try?