Open that file in a text program like Notepad or Wordpad and look at the beginning of that file.

A bookmarks backup JSON file will begin like this:

{"title":"","guid":"mJC706EZwTIU","id":1,"index":0,"dateAdded":1240105145406000,"lastModified":1328860824735000,"type":"text/x-moz-place-container","root":"placesRoot","children":[{"title":"Bookmarks Menu","

JSON file needs to be restored; it will replace any existing bookmarks with the bookmarks in that file.

A bookmarks.html file that was exported (or from a different brand browser) will begin like this.

<!DOCTYPE NETSCAPE-Bookmark-file-1>
<!-- This is an automatically generated file.
     It will be read and overwritten.
     DO NOT EDIT! -->
<META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=UTF-8">
<TITLE>Bookmarks</TITLE>
<H1>Bookmarks Menu</H1>
 

Bookmarks.HTML file needs to be imported; it will append any existing bookmarks - not replace any or all.

it is in JSON format. Do I need to change it to HTML to import? If so, how do I do that? Thanks!

Sorry, you can't just "change it to HTML". And I haven't seen a "converter" for doing a conversion of formats. The import and restore functions "look at" the file and make a determination if the file matches the file suffix. And an incorrectly formatted HTML file won't pass "muster".

What's the problem with doing a "restore" of that JSON file? If it because you want to 'move' those bookmarks into a Firefox installation that already has bookmarks that you don't want to overwrite, then export those bookmarks in HTML format and once the JSON backup is restored "import" that HTML bookmarks file.

I'm not sure I understand... I do not mind doing a "restore" of the JSON file that I backed up and saved. How do I restore it? (or, were you saying that I cannot restore that file)

FYI, when I try to restore it and select the option "choose a file" under the "Import and Backup" , the JSON file does not show up as an option either... Did I do the backup incorrectly?

See this support article. https://support.mozilla.org/en-US/kb/restore-bookmarks-from-backup-or-move-them

The file I saved to my external hardrive does not show up and only today's date is listed under the restore option files....

You can click the "Choose File" button at the bottom of the Restore list if you want to use a backup stored elsewhere. Make sure that the file has a .json file extension to make this work.