Hi newtowndaemon, and welcome!
Many thanks for pointing the way towards the gtk-bookmarks files.
I had to create the file
~/.config/gtk-3.0/bookmarks, then add the full file URL there.
After unveiling the folder, I quit, then restarted Firefox.
It worked!
See
GTK Bookmarks and
~/.gtk-bookmarks is deprecated for an explanation of the difference between gtk2 and gtk3 bookmarks.