I don't have your problem in Safari 10.0.3/Firefox 51.0.1; it will be helpful if you specify version numbers.
Mea culpa. Safari 6.1.6 and Firefox ESR 45.7.0.
What I can't figure out is how I was able (twice and apparently without difficulty) to import old Firefox Bookmarks a couple years back into Safari 5.1.7 (which I just updated to the version given above, which Software Update specifies as the latest update for Lion 10.7.5).