Firebug 1.5: Break On Next

The entire feature is represented by a new Break On button that is available in Firebug's main toolbar.

The logic of the button depends on the panel that is currently selected in Firebug. For instance, if the Script panel is selected and the feature activated (by clicking on the button that starts throbbing ), the debugger halts as soon as the next line of Javascript is executed. So, this is the moment when you have to e.g. press a button in your web app and see what code is actually called.

Here is how the button works in other Firebug panels (the button is disabled for panels, which doesn't support this feature):

• Script: Break on the next executed script.
• HTML: Break on HTML mutation (element addition or removal and attribute change).
• Net: Break on XMLHttpRequest execution.
• Console: Break on Javascript error.

http://www.softwareishard.com/blog/firebug/firebug-15-break-on-next/