Interesting idea, but forget Firefox, I don't see how website designers could target that either. For example, if you have the same buffer open in two windows but scrolled to different positions, what value will document.scrollingElement.scrollTop have? Similarly, different windows can have different dimensions, but webpages generally expect that there's only one viewport and adjust for that.

Previously, ctrl-shift-T only reopened tabs and worked per-window. I think it's much more intuitive that way, since it's cleanly separated from other features (ctrl-shift-N) and works kind of like ctrl-T.