This feature isn't yet supported in the new debugger. If you need to use it, you can, for now, switch back to the old debugger by visiting about:config and setting the "devtools.debugger.new-debugger-frontend" preference to false.
To see the docs for the old debugger, please visit Debugger (before Firefox 52).
If you're listening to a particular DOM event, you can tell the debugger to break when the event is triggered without having to track down the listener and set a breakpoint manually.
First, open the events pane by clicking the button in the toolbar that opens the shared variables/events pane. Then click the tab labeled "Events". The events pane will list all events for which you have assigned a listener:
Then check the box next to the event you want to break at.
When the event is triggered the code will break at the start of your listener.