To see what it's like in earlier versions of Firefox or in Firefox Beta and Release, see Debugger (before Firefox 52).
If you are getting this version of the Debugger and need to switch back to the old version, you can do so by visiting about:config and setting the "devtools.debugger.new-debugger-frontend" preference to
The button actually has three states, and clicking the button cycles from one state to the next.
: don't break on exceptions. This is the initial state.
: break only on uncaught exceptions.
: break on all exceptions.