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?
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 :(
On nyxt, if a windows tries to select a tab that is already selected on another tab, tab simply gets swapped between the two windows.