I think the fastest way to do this would be:

"From the keyboard: Press Ctrl + Shift + M (or Cmd + Opt + M on macOS)."

https://developer.mozilla.org/en-US/docs/Tools/Responsive_Design_Mode

It doesn't appear there's a way to start firefox in responsive design mode, even in FF Developer edition, unfortunately. If I find it, I'll update you.

Thanks for your answer, that's the way I already do it. It would be very nice to have such an option in FF Developer Edition allowing e.g. to simply type in the bash $ live-server --browser=firefox-dev to instantly get the page you are working on opened in responsive design mode + the Inspector.

John Smith said

I have some issues in the responsiveness of my site it's work properly on other browsers but not on that which is the way to make it responsive.

See my post (directed at you) in this thread :

https://support.mozilla.org/en-US/questions/1275427