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.
I've narrowed my tabs with userchrome.css so that they look like this. I think you could make the min-width even smaller so that the scrolling never happens, but then the tab icons won't be visible.
Can't bother debugging my userchrome right now to give a reproducible example but I think this is the relevant part:
.tabbrowser-tab[fadein]:not([pinned]) {
min-width: 30px !important;
padding-inline: 0 !important; /* not sure why I added this */
}