How to import Firefox bookmarks from old PC to new one?

I just discovered that my bookmarks in Mozilla Firefox is mostly empty.  I thought I had already managed to import or migrate them from my old HP PC (Windows 7) to my new Dell PC (Windows 10) in Firefox, but maybe I forgot or somehow made an error or mistake.  So I tried copying & pasting my old Firefox profile, but still nothing shows up in bookmarks.

Why not?  What am I doing wrong?  The "import bookmarks" function in Firefox doesn't seem to do anything either.  PLEASE HELP!