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
When the debugger is stopped at a breakpoint, you can step through it using four buttons in the toolbar:
In order, the buttons are:
- Play: run to the next breakpoint
- Step over: advance to the next line in the same function.
- Step into: advance to the next line in the function, unless on a function call, in which case enter the function being called
- Step out: run to the end of the current function