diff --git a/js/src/js.msg b/js/src/js.msg
index b979b6e74e5c..f08a05ea77d3 100644
--- a/js/src/js.msg
+++ b/js/src/js.msg
@@ -313,7 +313,7 @@ MSG_DEF(JSMSG_CANT_CHANGE_EXTENSIBILITY, 259, 0, JSEXN_TYPEERR, "can't change ob
 MSG_DEF(JSMSG_SC_BAD_SERIALIZED_DATA, 260, 1, JSEXN_INTERNALERR, "bad serialized structured data ({0})")
 MSG_DEF(JSMSG_SC_UNSUPPORTED_TYPE,    261, 0, JSEXN_TYPEERR, "unsupported type for structured data")
 MSG_DEF(JSMSG_SC_RECURSION,           262, 0, JSEXN_INTERNALERR, "recursive object")
-MSG_DEF(JSMSG_UNUSED263,              263, 0, JSEXN_NONE, "")
+MSG_DEF(JSMSG_DEBUG_CANT_DEBUG_GLOBAL, 263, 0, JSEXN_ERR, "passing non-debuggable global to addDebuggee")
 MSG_DEF(JSMSG_BAD_CLONE_VERSION,      264, 0, JSEXN_ERR, "unsupported structured clone version")
 MSG_DEF(JSMSG_CANT_CLONE_OBJECT,      265, 0, JSEXN_TYPEERR, "can't clone object")
 MSG_DEF(JSMSG_DEBUG_RESUMPTION_VALUE_DISALLOWED, 266, 0, JSEXN_TYPEERR, "resumption values are disallowed in this hook")
diff --git a/js/src/jsapi.h b/js/src/jsapi.h
index b2d0a45b7a41..7da0c0f8260e 100644
--- a/js/src/jsapi.h
+++ b/js/src/jsapi.h
@@ -3223,9 +3223,11 @@ SameZoneAs(JSObject *obj)
 struct JS_PUBLIC_API(CompartmentOptions) {
     ZoneSpecifier zoneSpec;
     JSVersion version;
+    bool invisibleToDebugger;
 
     explicit CompartmentOptions() : zoneSpec(JS::FreshZone)
                                   , version(JSVERSION_UNKNOWN)
+                                  , invisibleToDebugger(false)
     {}
 
     CompartmentOptions &setZone(ZoneSpecifier spec) { zoneSpec = spec; return *this; }
@@ -3234,6 +3236,15 @@ struct JS_PUBLIC_API(CompartmentOptions) {
         version = version_;
         return *this;
     }
+
+    // Certain scopes (i.e. XBL compilation scopes) are implementation details
+    // of the embedding, and references to them should never leak out to script.
+    // This flag causes the this compartment to skip firing onNewGlobalObject
+    // and makes addDebuggee a no-op for this global.
+    CompartmentOptions &setInvisibleToDebugger(bool invisible) {
+        invisibleToDebugger = invisible;
+        return *this;
+    }
 };
 
 } /* namespace JS */
diff --git a/js/src/vm/Debugger.cpp b/js/src/vm/Debugger.cpp
index 806092e3fa32..00b2f5c71ec4 100644
--- a/js/src/vm/Debugger.cpp
+++ b/js/src/vm/Debugger.cpp
@@ -1339,6 +1339,8 @@ void
 Debugger::slowPathOnNewGlobalObject(JSContext *cx, Handle<GlobalObject *> global)
 {
     JS_ASSERT(!JS_CLIST_IS_EMPTY(&cx->runtime()->onNewGlobalObjectWatchers));
+    if (global->compartment()->options().invisibleToDebugger)
+        return;
 
     /*
      * Make a copy of the runtime's onNewGlobalObjectWatchers before running the
@@ -1958,7 +1960,7 @@ Debugger::addAllGlobalsAsDebuggees(JSContext *cx, unsigned argc, Value *vp)
     THIS_DEBUGGER(cx, argc, vp, "addAllGlobalsAsDebuggees", args, dbg);
     AutoDebugModeGC dmgc(cx->runtime());
     for (CompartmentsIter c(cx->runtime()); !c.done(); c.next()) {
-        if (c == dbg->object->compartment())
+        if (c == dbg->object->compartment() || c->options().invisibleToDebugger)
             continue;
         c->zone()->scheduledForDestruction = false;
         GlobalObject *global = c->maybeGlobal();
@@ -2134,7 +2136,16 @@ Debugger::addDebuggeeGlobal(JSContext *cx,
     if (debuggees.has(global))
         return true;
 
+    // Callers should generally be unable to get a reference to a debugger-
+    // invisible global in order to pass it to addDebuggee. But this is possible
+    // with certain testing aides we expose in the shell, so just make addDebuggee
+    // throw in that case.
     JSCompartment *debuggeeCompartment = global->compartment();
+    if (debuggeeCompartment->options().invisibleToDebugger) {
+        JS_ReportErrorNumber(cx, js_GetErrorMessage, NULL,
+                             JSMSG_DEBUG_CANT_DEBUG_GLOBAL);
+        return false;
+    }
 
     /*
      * Check for cycles. If global's compartment is reachable from this