Open the debugger


  • select "Debugger" from the Web Developer submenu in the Firefox Menu (or Tools menu if you display the menu bar or are on Mac OS X)
  • press the Ctrl + Shift + S on Windows and Linux or Cmd + Opt + S on macOS

    Note: This shortcut will not work with Firefox 66 or later. However, you can press Ctrl + Shift + I on Windows and Linux, Cmd + Opt + I, or F12 to open the toolbox and then select the Debugger tool.
  • press the menu button ( new fx menu ), select "Developer", then "Debugger".