Set a breakpoint on line 58 (var closure = getClosure();)
Reloaded the page => Script execution stopped at line 58.
Clicked 'Step Into' => Script execution pointer moved into the getClosure() function on line 48.
Do you have a reproducible test case?
Note that the guys behind Firebug are focusing their work on Firebug 3, which will integrate into the Firefox DevTools. Maintenance on Firebug 2 is mostly shut down.