diff --git a/toolkit/devtools/server/child.js b/toolkit/devtools/server/child.js
index a5553507c32a..6e296fa9402d 100644
--- a/toolkit/devtools/server/child.js
+++ b/toolkit/devtools/server/child.js
@@ -26,18 +26,14 @@
 
     let mm = msg.target;
 
-    let prefix = msg.data.prefix + docShell.appId;
-
-    let conn = DebuggerServer.connectToParent(prefix, mm);
+    let conn = DebuggerServer.connectToParent(msg.data.prefix, mm);
 
     let actor = new DebuggerServer.ContentAppActor(conn, content);
     let actorPool = new ActorPool(conn);
     actorPool.addActor(actor);
     conn.addActorPool(actorPool);
 
-    sendAsyncMessage("debug:actor", {actor: actor.grip(),
-                                     appId: docShell.appId,
-                                     prefix: prefix});
+    sendAsyncMessage("debug:actor", {actor: actor.grip()});
   });
 
   addMessageListener("debug:connect", onConnect);
diff --git a/toolkit/devtools/server/main.js b/toolkit/devtools/server/main.js
index c6d7b2ed1bfb..397ded1d1a0e 100644
--- a/toolkit/devtools/server/main.js
+++ b/toolkit/devtools/server/main.js
@@ -531,13 +531,12 @@ var DebuggerServer = {
     let mm = aMessageManager;
     mm.loadFrameScript("resource://gre/modules/devtools/server/child.js", false);
 
-    let actor, childTransport, prefix;
+    let actor, childTransport;
+    let prefix = aConnection.allocID("child");
 
     let onActorCreated = DevToolsUtils.makeInfallible(function (msg) {
       mm.removeMessageListener("debug:actor", onActorCreated);
 
-      prefix = msg.json.prefix;
-
       // Pipe Debugger message from/to parent/child via the message manager
       childTransport = new ChildDebuggerTransport(mm, prefix);
       childTransport.hooks = {
@@ -586,8 +585,7 @@ var DebuggerServer = {
     Services.obs.addObserver(onMessageManagerDisconnect,
                              "message-manager-disconnect", false);
 
-    let prefixStart = aConnection.prefix + "child";
-    mm.sendAsyncMessage("debug:connect", { prefix: prefixStart });
+    mm.sendAsyncMessage("debug:connect", { prefix: prefix });
 
     return deferred.promise;
   },