Both the menu items and the tabs are tied to their own dedicated toolbar and can't be moved to another toolbar. Both allow other items to be placed on their toolbar and you can use an extension to subsequently hide empty toolbar if you can't do this in Firefox.

You would need to use an extension that either makes it possible to merge the two toolbars or collapse the menu items in a toolbar button and place that button on the Tab bar.

Why even have the Menu bar showing all the time? It is easily displayed via the Alt key or the F10 key.