Firefox still has a "flexible space" for the toolbars, but no way for the user to access it to drag it to the toolbars. If you would install the Classic Theme Restorer extension (temporarily) and then drag a flexible space to each end of the toolbar, you will be able to make use of the "flexible space" as many time as you need to. Then uninstall that CTR extension; the "flexible spaces" should "stick" where you placed them. Probably won't work too well or at all on the Bookmarks Toolbar, though.

For the Bookmarks Toolbar you only need to place a flexible space at the start (i.e. before the Bookmarks Toolbar items) because there is already a flexible space at the end.