this is duly noted. @nchevobbe replied some directions in the mozilla chat as to how i can setup, and as im really new, will try to familiarize with the workflow first. thanks for guidance. 
For reference, @maurusrv created an issue for this to discuss its UX:
And btw., @frode, note that there’s already a feature request for showing the viewport dimensions when resizing the window.
Sebastian
Wow, I see this discussion has been going on for a while and Chrome is going ahead. I’d like to be able to see media queries in responsive design view as well, not only to check my layouts but also to check which of my integrated framework CSS files may use other queries.
Me too! I’d really like to see this function implemented.
Also, another borrowed from Edge instead: css selector profiler!