diff --git a/js/src/jit-test/jit_test.py b/js/src/jit-test/jit_test.py
index cf8ec1ece055..72ea5fd6eb60 100755
--- a/js/src/jit-test/jit_test.py
+++ b/js/src/jit-test/jit_test.py
@@ -95,6 +95,8 @@ class Test:
                         test.valgrind = options.valgrind
                     elif name == 'mjitalways':
                         test.jitflags.append('-a')
+                    elif name == 'debug':
+                        test.jitflags.append('-d')
                     else:
                         print('warning: unrecognized |jit-test| attribute %s'%part)
 
diff --git a/js/src/jit-test/tests/basic/testBug552248.js b/js/src/jit-test/tests/basic/testBug552248.js
index 28782b665bb3..94aef6a148a5 100644
--- a/js/src/jit-test/tests/basic/testBug552248.js
+++ b/js/src/jit-test/tests/basic/testBug552248.js
@@ -1,3 +1,4 @@
+// |jit-test| debug
 setDebug(true);
 var a = new Array();
 
diff --git a/js/src/jit-test/tests/jaeger/bug563000/eif-call-newvar.js b/js/src/jit-test/tests/jaeger/bug563000/eif-call-newvar.js
index 59b0dfb19ac9..eafcaa07610c 100644
--- a/js/src/jit-test/tests/jaeger/bug563000/eif-call-newvar.js
+++ b/js/src/jit-test/tests/jaeger/bug563000/eif-call-newvar.js
@@ -1,4 +1,4 @@
-// |jit-test| mjitalways
+// |jit-test| mjitalways;debug
 setDebug(true);
 
 function callee() {
diff --git a/js/src/jit-test/tests/jaeger/bug563000/eif-call-typechange.js b/js/src/jit-test/tests/jaeger/bug563000/eif-call-typechange.js
index 3870b57dea9f..238dbeedc340 100644
--- a/js/src/jit-test/tests/jaeger/bug563000/eif-call-typechange.js
+++ b/js/src/jit-test/tests/jaeger/bug563000/eif-call-typechange.js
@@ -1,4 +1,4 @@
-// |jit-test| mjitalways
+// |jit-test| mjitalways;debug
 setDebug(true);
 
 function callee() {
diff --git a/js/src/jit-test/tests/jaeger/bug563000/eif-call.js b/js/src/jit-test/tests/jaeger/bug563000/eif-call.js
index b28c66a8d7e3..73dd3674a6fd 100644
--- a/js/src/jit-test/tests/jaeger/bug563000/eif-call.js
+++ b/js/src/jit-test/tests/jaeger/bug563000/eif-call.js
@@ -1,4 +1,4 @@
-// |jit-test| mjitalways
+// |jit-test| mjitalways;debug
 setDebug(true);
 
 function callee() {
diff --git a/js/src/jit-test/tests/jaeger/bug563000/eif-getter-newvar.js b/js/src/jit-test/tests/jaeger/bug563000/eif-getter-newvar.js
index 21fbbf7f52cb..d55e706a9839 100644
--- a/js/src/jit-test/tests/jaeger/bug563000/eif-getter-newvar.js
+++ b/js/src/jit-test/tests/jaeger/bug563000/eif-getter-newvar.js
@@ -1,4 +1,4 @@
-// |jit-test| mjitalways
+// |jit-test| mjitalways;debug
 setDebug(true);
 
 this.__defineGetter__("someProperty", function () { evalInFrame(1, "var x = 'success'"); });
diff --git a/js/src/jit-test/tests/jaeger/bug563000/eif-getter-typechange.js b/js/src/jit-test/tests/jaeger/bug563000/eif-getter-typechange.js
index 0e9480d369ad..914b19c23e15 100644
--- a/js/src/jit-test/tests/jaeger/bug563000/eif-getter-typechange.js
+++ b/js/src/jit-test/tests/jaeger/bug563000/eif-getter-typechange.js
@@ -1,4 +1,4 @@
-// |jit-test| mjitalways
+// |jit-test| mjitalways;debug
 setDebug(true);
 
 this.__defineGetter__("someProperty", function () { evalInFrame(1, "var x = 'success'"); });
diff --git a/js/src/jit-test/tests/jaeger/bug563000/eif-getter.js b/js/src/jit-test/tests/jaeger/bug563000/eif-getter.js
index 5127beacd7f7..a1631ccd36cb 100644
--- a/js/src/jit-test/tests/jaeger/bug563000/eif-getter.js
+++ b/js/src/jit-test/tests/jaeger/bug563000/eif-getter.js
@@ -1,4 +1,4 @@
-// |jit-test| mjitalways
+// |jit-test| mjitalways;debug
 setDebug(true);
 
 this.__defineGetter__("someProperty", function () { evalInFrame(1, "x = 'success'"); });
diff --git a/js/src/jit-test/tests/jaeger/bug563000/eif-trap-newvar.js b/js/src/jit-test/tests/jaeger/bug563000/eif-trap-newvar.js
index aa2906db36e6..fb048587faf9 100644
--- a/js/src/jit-test/tests/jaeger/bug563000/eif-trap-newvar.js
+++ b/js/src/jit-test/tests/jaeger/bug563000/eif-trap-newvar.js
@@ -1,4 +1,4 @@
-// |jit-test| mjitalways
+// |jit-test| mjitalways;debug
 setDebug(true);
 
 function nop(){}
diff --git a/js/src/jit-test/tests/jaeger/bug563000/eif-trap-typechange.js b/js/src/jit-test/tests/jaeger/bug563000/eif-trap-typechange.js
index 3caa1d3160d2..4028293f08b8 100644
--- a/js/src/jit-test/tests/jaeger/bug563000/eif-trap-typechange.js
+++ b/js/src/jit-test/tests/jaeger/bug563000/eif-trap-typechange.js
@@ -1,4 +1,4 @@
-// |jit-test| mjitalways
+// |jit-test| mjitalways;debug
 setDebug(true);
 
 function nop(){}
diff --git a/js/src/jit-test/tests/jaeger/bug563000/eif-trap.js b/js/src/jit-test/tests/jaeger/bug563000/eif-trap.js
index 1d54ec7341c1..43d013da9358 100644
--- a/js/src/jit-test/tests/jaeger/bug563000/eif-trap.js
+++ b/js/src/jit-test/tests/jaeger/bug563000/eif-trap.js
@@ -1,4 +1,4 @@
-// |jit-test| mjitalways
+// |jit-test| mjitalways;debug
 setDebug(true);
 
 function nop(){}
diff --git a/js/src/jit-test/tests/jaeger/bug563000/simple-trap-1.js b/js/src/jit-test/tests/jaeger/bug563000/simple-trap-1.js
index b42c84fa8454..ef77a86e5304 100644
--- a/js/src/jit-test/tests/jaeger/bug563000/simple-trap-1.js
+++ b/js/src/jit-test/tests/jaeger/bug563000/simple-trap-1.js
@@ -1,3 +1,4 @@
+// |jit-test| debug
 setDebug(true);
 var x = "failure";
 function main() { x = "success"; }
diff --git a/js/src/jit-test/tests/jaeger/bug563000/simple-trap-2.js b/js/src/jit-test/tests/jaeger/bug563000/simple-trap-2.js
index 675fdcd79a02..c12fdff984c6 100644
--- a/js/src/jit-test/tests/jaeger/bug563000/simple-trap-2.js
+++ b/js/src/jit-test/tests/jaeger/bug563000/simple-trap-2.js
@@ -1,3 +1,4 @@
+// |jit-test| debug
 setDebug(true);
 var x = "notset";
 function main() { x = "failure"; }
diff --git a/js/src/jit-test/tests/jaeger/bug563000/simple-untrap.js b/js/src/jit-test/tests/jaeger/bug563000/simple-untrap.js
index 2f89f58bc3dd..687802127162 100644
--- a/js/src/jit-test/tests/jaeger/bug563000/simple-untrap.js
+++ b/js/src/jit-test/tests/jaeger/bug563000/simple-untrap.js
@@ -1,3 +1,4 @@
+// |jit-test| debug
 setDebug(true);
 var x = "notset";
 function main() { x = "success"; }
diff --git a/js/src/jit-test/tests/jaeger/bug563000/test-debugger-1.js b/js/src/jit-test/tests/jaeger/bug563000/test-debugger-1.js
index 19c76feb1536..5ba1bf681685 100644
--- a/js/src/jit-test/tests/jaeger/bug563000/test-debugger-1.js
+++ b/js/src/jit-test/tests/jaeger/bug563000/test-debugger-1.js
@@ -1,3 +1,4 @@
+// |jit-test| debug
 var result = "unset";
 function main() {
   result = "failure";
diff --git a/js/src/jit-test/tests/jaeger/bug563000/test-debugger-2.js b/js/src/jit-test/tests/jaeger/bug563000/test-debugger-2.js
index ff3b9e3185e2..291d1819da11 100644
--- a/js/src/jit-test/tests/jaeger/bug563000/test-debugger-2.js
+++ b/js/src/jit-test/tests/jaeger/bug563000/test-debugger-2.js
@@ -1,3 +1,4 @@
+// |jit-test| debug
 function main() {
   debugger;
   return "failure";
diff --git a/js/src/jit-test/tests/jaeger/bug563000/test-throwhook-1.js b/js/src/jit-test/tests/jaeger/bug563000/test-throwhook-1.js
index 9e85e26cef52..5288ab39ee3a 100644
--- a/js/src/jit-test/tests/jaeger/bug563000/test-throwhook-1.js
+++ b/js/src/jit-test/tests/jaeger/bug563000/test-throwhook-1.js
@@ -1,3 +1,4 @@
+// |jit-test| debug
 var result1 = "unset";
 var result2 = "failure";
 function main() {
diff --git a/js/src/jit-test/tests/jaeger/bug563000/test-throwhook-2.js b/js/src/jit-test/tests/jaeger/bug563000/test-throwhook-2.js
index 053f37aeffda..cf2b08d87f2a 100644
--- a/js/src/jit-test/tests/jaeger/bug563000/test-throwhook-2.js
+++ b/js/src/jit-test/tests/jaeger/bug563000/test-throwhook-2.js
@@ -1,3 +1,4 @@
+// |jit-test| debug
 function main() {
   try {
     throw "something";
diff --git a/js/src/jit-test/tests/jaeger/bug563000/trap-force-return-1.js b/js/src/jit-test/tests/jaeger/bug563000/trap-force-return-1.js
index 1d19aeebeb11..09ae0db40423 100644
--- a/js/src/jit-test/tests/jaeger/bug563000/trap-force-return-1.js
+++ b/js/src/jit-test/tests/jaeger/bug563000/trap-force-return-1.js
@@ -1,3 +1,4 @@
+// |jit-test| debug
 setDebug(true);
 function main() {
   return "failure";
diff --git a/js/src/jit-test/tests/jaeger/bug563000/trap-force-return-2.js b/js/src/jit-test/tests/jaeger/bug563000/trap-force-return-2.js
index b2b516826a9e..b318e79f11bc 100644
--- a/js/src/jit-test/tests/jaeger/bug563000/trap-force-return-2.js
+++ b/js/src/jit-test/tests/jaeger/bug563000/trap-force-return-2.js
@@ -1,3 +1,4 @@
+// |jit-test| debug
 setDebug(true);
 function main() {
   return 1;
diff --git a/js/src/jit-test/tests/jaeger/bug563000/trap-from-add-inline.js b/js/src/jit-test/tests/jaeger/bug563000/trap-from-add-inline.js
index f923504d344a..d66c0c977c92 100644
--- a/js/src/jit-test/tests/jaeger/bug563000/trap-from-add-inline.js
+++ b/js/src/jit-test/tests/jaeger/bug563000/trap-from-add-inline.js
@@ -1,3 +1,4 @@
+// |jit-test| debug
 setDebug(true);
 x = "notset";
 function main() {
diff --git a/js/src/jit-test/tests/jaeger/bug563000/trap-from-add-ool.js b/js/src/jit-test/tests/jaeger/bug563000/trap-from-add-ool.js
index 48eb377f52cd..5f0f41b6be20 100644
--- a/js/src/jit-test/tests/jaeger/bug563000/trap-from-add-ool.js
+++ b/js/src/jit-test/tests/jaeger/bug563000/trap-from-add-ool.js
@@ -1,3 +1,4 @@
+// |jit-test| debug
 setDebug(true);
 x = "notset";
 function main() {
diff --git a/js/src/jit-test/tests/jaeger/bug563000/trap-own-callsite.js b/js/src/jit-test/tests/jaeger/bug563000/trap-own-callsite.js
index 18378b7808f7..319ca3680d89 100644
--- a/js/src/jit-test/tests/jaeger/bug563000/trap-own-callsite.js
+++ b/js/src/jit-test/tests/jaeger/bug563000/trap-own-callsite.js
@@ -1,3 +1,4 @@
+// |jit-test| debug
 setDebug(true);
 x = "notset";
 function myparent(nested) {
diff --git a/js/src/jit-test/tests/jaeger/bug563000/trap-parent-from-trap.js b/js/src/jit-test/tests/jaeger/bug563000/trap-parent-from-trap.js
index d6ded288128c..2b4653e7cadc 100644
--- a/js/src/jit-test/tests/jaeger/bug563000/trap-parent-from-trap.js
+++ b/js/src/jit-test/tests/jaeger/bug563000/trap-parent-from-trap.js
@@ -1,3 +1,4 @@
+// |jit-test| debug
 setDebug(true);
 x = "notset";
 
diff --git a/js/src/jit-test/tests/jaeger/bug563000/trap-parent.js b/js/src/jit-test/tests/jaeger/bug563000/trap-parent.js
index 2209f946af97..2baeb08b627f 100644
--- a/js/src/jit-test/tests/jaeger/bug563000/trap-parent.js
+++ b/js/src/jit-test/tests/jaeger/bug563000/trap-parent.js
@@ -1,3 +1,4 @@
+// |jit-test| debug
 setDebug(true);
 x = "notset";
 function child() {
diff --git a/js/src/jit-test/tests/jaeger/bug563000/trap-self-as-parent.js b/js/src/jit-test/tests/jaeger/bug563000/trap-self-as-parent.js
index 65d3e73135e6..16c9bd0a9403 100644
--- a/js/src/jit-test/tests/jaeger/bug563000/trap-self-as-parent.js
+++ b/js/src/jit-test/tests/jaeger/bug563000/trap-self-as-parent.js
@@ -1,3 +1,4 @@
+// |jit-test| debug
 setDebug(true);
 x = "notset";
 
diff --git a/js/src/jit-test/tests/jaeger/bug563000/trap-self-from-trap.js b/js/src/jit-test/tests/jaeger/bug563000/trap-self-from-trap.js
index 8a9caaaafda4..ef5b3f53a97b 100644
--- a/js/src/jit-test/tests/jaeger/bug563000/trap-self-from-trap.js
+++ b/js/src/jit-test/tests/jaeger/bug563000/trap-self-from-trap.js
@@ -1,3 +1,4 @@
+// |jit-test| debug
 setDebug(true);
 x = "notset";
 
diff --git a/js/src/jit-test/tests/jaeger/bug563000/trap-self.js b/js/src/jit-test/tests/jaeger/bug563000/trap-self.js
index 9b94fd1c8cca..5f8e11e97a45 100644
--- a/js/src/jit-test/tests/jaeger/bug563000/trap-self.js
+++ b/js/src/jit-test/tests/jaeger/bug563000/trap-self.js
@@ -1,3 +1,4 @@
+// |jit-test| debug
 setDebug(true);
 x = "notset";
 function main() {
diff --git a/js/src/jit-test/tests/jaeger/bug563000/untrap-own-trapsite.js b/js/src/jit-test/tests/jaeger/bug563000/untrap-own-trapsite.js
index e4dd9d7dc973..db5c2f50b792 100644
--- a/js/src/jit-test/tests/jaeger/bug563000/untrap-own-trapsite.js
+++ b/js/src/jit-test/tests/jaeger/bug563000/untrap-own-trapsite.js
@@ -1,3 +1,4 @@
+// |jit-test| debug
 setDebug(true);
 x = "notset";
 function child() {
diff --git a/js/src/jit-test/tests/jaeger/bug563000/untrap-self.js b/js/src/jit-test/tests/jaeger/bug563000/untrap-self.js
index 4c18db52e8ed..b4b80d1a8dfe 100644
--- a/js/src/jit-test/tests/jaeger/bug563000/untrap-self.js
+++ b/js/src/jit-test/tests/jaeger/bug563000/untrap-self.js
@@ -1,3 +1,4 @@
+// |jit-test| debug
 setDebug(true);
 x = "notset";
 function main() {
diff --git a/js/src/jsdbgapi.cpp b/js/src/jsdbgapi.cpp
index 31e007148f19..dc8dc803eda9 100644
--- a/js/src/jsdbgapi.cpp
+++ b/js/src/jsdbgapi.cpp
@@ -145,6 +145,9 @@ CompartmentHasLiveScripts(JSCompartment *comp)
 JS_FRIEND_API(JSBool)
 JS_SetDebugModeForCompartment(JSContext *cx, JSCompartment *comp, JSBool debug)
 {
+    if (comp->debugMode == debug)
+        return JS_TRUE;
+
     // This should only be called when no scripts are live. It would even be
     // incorrect to discard just the non-live scripts' JITScripts because they
     // might share ICs with live scripts (bug 632343).
diff --git a/js/src/shell/js.cpp b/js/src/shell/js.cpp
index 9605ba770829..eeb0f892fff1 100644
--- a/js/src/shell/js.cpp
+++ b/js/src/shell/js.cpp
@@ -1745,9 +1745,16 @@ SetDebug(JSContext *cx, uintN argc, jsval *vp)
                              JSSMSG_NOT_ENOUGH_ARGS, "setDebug");
         return JS_FALSE;
     }
+
+    /*
+     * Debug mode can only be set when there is no JS code executing on the
+     * stack. Unfortunately, that currently means that this call will fail
+     * unless debug mode is already set to what you're trying to set it to.
+     * In the future, this restriction may be lifted.
+     */
     
-    JS_SetDebugMode(cx, JSVAL_TO_BOOLEAN(argv[0]));
-    JS_SET_RVAL(cx, vp, JSVAL_VOID);
+    JSBool rv = JS_SetDebugMode(cx, JSVAL_TO_BOOLEAN(argv[0]));
+    JS_SET_RVAL(cx, vp, rv ? JSVAL_TRUE : JSVAL_FALSE);
     return JS_TRUE;
 }