You clearly don’t use Firefox much… that’s the Bookmark Toolbar!

The mysterious blank bar is blank because it is the bookmarks bar. If
you are not using it you can always turn it off via View -> Toolbars ->
Bookmarks Toolbar. Doesn’t IE have the same thing? I would think for
someone to do a fair comparison of two browsers’ UI that person would
actually have to be accustomed to using both browsers. Disappointing
and sloppy work regarding something so simple.