The Tor Browser / file revision

 // Clearing a frame's onPop handler works.

 var g = newGlobal();

 g.eval("function f() { debugger; }");

 var dbg = new Debugger(g);

 var log;

 dbg.onEnterFrame = function handleEnter(f) {

     log += "(";

     f.onPop = function handlePop() {

         assertEq("handlePop was called", "handlePop should never be called");

     };

 };

 dbg.onDebuggerStatement = function handleDebugger(f) {

     log += "d";

     assertEq(typeof f.onPop, "function");

     f.onPop = undefined;

 };

 log = '';

 g.f();

 assertEq(log, "(d");