Breaking on exceptions

Firefox Version 64

To instruct the debugger to pause on an exception, click on these checkboxes in the Breakpoints list.
 
Screen shot from 64.0+build3-0ubuntu0.16.04.1
 

Earlier Versions

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.