--- a/js/src/jsdbg.cpp
+++ b/js/src/jsdbg.cpp
@@ -856,24 +856,25 @@ Debug::getDebuggees(JSContext *cx, uintN
     vp->setObject(*arrobj);
     return true;
 }
 
 JSBool
 Debug::getYoungestFrame(JSContext *cx, uintN argc, Value *vp)
 {
     THISOBJ(cx, vp, Debug, "getYoungestFrame", thisobj, dbg);
-    StackFrame *fp = cx->fp();
-    while (fp && !dbg->observesFrame(fp))
-        fp = fp->prev();
-    if (!fp) {
-        vp->setNull();
-        return true;
+
+    // cx->fp() would return the topmost frame in the current context.
+    // Since there may be multiple contexts, use AllFramesIter instead.
+    for (AllFramesIter i(cx); !i.done(); ++i) {
+        if (dbg->observesFrame(i.fp()))
+            return dbg->getScriptFrame(cx, i.fp(), vp);
     }
-    return dbg->getScriptFrame(cx, fp, vp);
+    vp->setNull();
+    return true;
 }
 
 JSBool
 Debug::construct(JSContext *cx, uintN argc, Value *vp)
 {
     // Check that the arguments, if any, are cross-compartment wrappers.
     Value *argv = vp + 2, *argvEnd = argv + argc;
     for (Value *p = argv; p != argvEnd; p++) {
@@ -1087,22 +1088,34 @@ CheckThisFrame(JSContext *cx, Value *vp,
             JS_ReportErrorNumber(cx, js_GetErrorMessage, NULL, JSMSG_DEBUG_FRAME_NOT_LIVE,
                                  "Debug.Frame", fnname);
             return NULL;
         }
     }
     return thisobj;
 }
 
+#if DEBUG
+static bool
+StackContains(JSContext *cx, StackFrame *fp)
+{
+    for (AllFramesIter i(cx); !i.done(); ++i) {
+        if (fp == i.fp())
+            return true;
+    }
+    return false;
+}
+#endif
+
 #define THIS_FRAME(cx, vp, fnname, thisobj, fp)                              \
     JSObject *thisobj = CheckThisFrame(cx, vp, fnname, true);                \
     if (!thisobj)                                                            \
         return false;                                                        \
     StackFrame *fp = (StackFrame *) thisobj->getPrivate();                   \
-    JS_ASSERT((cx)->stack.contains(fp))
+    JS_ASSERT(StackContains(cx, fp))
 
 static JSBool
 DebugFrame_getType(JSContext *cx, uintN argc, Value *vp)
 {
     THIS_FRAME(cx, vp, "get type", thisobj, fp);
 
     // Indirect eval frames are both isGlobalFrame() and isEvalFrame(), so the
     // order of checks here is significant.