Many thanks to you all in this thread.
I've just updated Firefox to v57 on my laptop after a re-installation of Windows 10 and having the tabs below the Bookmarks toolbar has always been a strong preference for me. I found this thread after doing a Google search re Classic Theme Restorer for v57
I tried Reflective's code first but I found that, although the tabs had moved down, the bookmarks had moved up to immediately below the Menu bar and above the Navigation bar! Unfortunately, I am not familiar enough with coding to see why.
I have found the solution using Eckard's version of code, plus the additional code for the rounded tabs corners, so thank you so much, Eckard, for posting your code. Absolutely brilliant.
So, thanks to the original poster for instigating this thread and thanks to all the other posters for contributing to such an effective end, but especially Reflective and Eckard, of course