If you open a new windows in Nyxt browser, you'll notice that it will have the exact same tabs as the old window.
Same can be done with Emacs when in server mode: frames (windows) do share the same set of buffers.
I think this is a comfy feature, since windows' purpose is - IME - not to logically group tabs but rather only to give multi pane visualization when browsing.
So the question is: Is it possible to achieve this in Firefox?
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.
I think that's going to be the job of an extension if it can be done. Although I don't know if Firefox can have multiple viewports onto the same document object model.
Certainly if it's possible it will not be straightforward to accomplish, sorry to say :(