There are three ways to toggle Responsive Design Mode:

1. From the Firefox menu: Select Responsive Design Mode from the Web Developer submenu in the Firefox Menu (or Tools menu if you display the menu bar or are on macOS).
2. From the Developer Tools toolbox: Press the Responsive Design Mode button in the Toolbox’s toolbar.
3. From the keyboard: Press Ctrl+Shift+M (or Cmd+Opt+M on macOS).

1. "Responsive Design Mode" - Firefox Source Docs