This page describes the JavaScript Debugger as it appears in Firefox 52 and above, in Firefox Nightly and Firefox Developer Edition.
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 false.
To instruct the debugger to pause on an exception, click this icon: 
in the toolbar.
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.