If you're using Firefox 52 or later and need to switch back to the old Debugger, you can do so by visiting about:config and setting the "devtools.debugger.new-debugger-frontend" preference to
There are three ways to 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 CtrlShiftS (CommandOptionS on OS X) keyboard shortcut
- press the menu button ( ), press "Developer", then select "Debugger".