Several years I use firefox on 4K resolution without scaling. Opened 3 vertical windows with 1\3 width and full height. Close all windows at exit with Ctrl+Shift+Q, launch by shortcut usually opens all 3 windows on it's places.
Sometimes the sizes was changed by some reason - i have to adjust sizes back manually (use reSizer v0.78).
Also i cannot close all windows just by clicking standard window close button, because it closes single window with lost of their tabs (restorable). Warning at close tabs option in settings started to annoy me at every exit, it didn’t happen before, so I refused it turning on.
Main idea - make a layout layer, that handle all opened windows in it, store its proportional sizes and positions and closes all layout on close a single window (that in layout). Also should be possibility to maximize a single window in layout. I understand that ideally there is a lot of work, with that layer.. but I think, just linking several windows may be enough for start with.