Try this: Import Bookmarks into Firefox
On my machine the function is labeled 'import and backup' and it's in the menu bar at the top of the box. but it does the same thing. It might be different if you haven't saved any bookmarks (favorites) to FF yet because that ctrl+shift+B technically opens the bookmarks thingie.
I'll check it on my Win10 machine eventually to see what happens there.
Something else to add to my FF. I’m reaching the end of my productiveness for this day. I wish I had started this sooner...
*sigh*