It would be great if two websites could be opened in the same Firefox window instead of having to open two Firefox window and place them side by side on the screen.
At first I wasn't sure what you meant. So you want to be able to view two tabs side by side will assigned tot he same window. Out of curiosity what advantage does this provide over window groups or virtual desktops?
I could open another window but do find it more convenient to be able to open more that one site in a single Firefox window. For example, when I am managing my financial transactions across different bank accounts. Just personal preference.
it is named tiling and it is already managed by operating systems like Pop!_OS or many already existing apps that will always be much more efficient than Firefox getting fat trying to do it