--- a/toolkit/components/extensions/Extension.jsm
+++ b/toolkit/components/extensions/Extension.jsm
@@ -322,21 +322,16 @@ class ProxyContext extends BaseContext {
 class ExtensionChildProxyContext extends ProxyContext {
   constructor(envType, extension, params, xulBrowser) {
     super(envType, extension, params, xulBrowser, extension.principal);
 
     this.viewType = params.viewType;
     // WARNING: The xulBrowser may change when docShells are swapped, e.g. when
     // the tab moves to a different window.
     this.xulBrowser = xulBrowser;
-
-    // TODO(robwu): Remove this once all APIs can run in a separate process.
-    if (params.cloneScopeInProcess) {
-      this.sandbox = params.cloneScopeInProcess;
-    }
   }
 
   // The window that contains this context. This may change due to moving tabs.
   get xulWindow() {
     return this.xulBrowser.ownerGlobal;
   }
 
   get windowId() {

--- a/toolkit/components/extensions/ExtensionChild.jsm
+++ b/toolkit/components/extensions/ExtensionChild.jsm
@@ -78,26 +78,36 @@ var apiManager = new class extends Schem
 class WannabeChildAPIManager extends ChildAPIManager {
   createProxyContextInConstructor(data) {
     // Create a structured clone to simulate IPC.
     data = Object.assign({}, data);
     let {principal} = data;  // Not structurally cloneable.
     delete data.principal;
     data = Cu.cloneInto(data, {});
     data.principal = principal;
-    data.cloneScopeInProcess = this.context.cloneScope;
     let name = "API:CreateProxyContext";
     // The <browser> that receives messages from `this.messageManager`.
     let target = this.context.contentWindow
       .QueryInterface(Ci.nsIInterfaceRequestor)
       .getInterface(Ci.nsIDocShell)
       .chromeEventHandler;
     ParentAPIManager.receiveMessage({name, data, target});
 
     let proxyContext = ParentAPIManager.proxyContexts.get(this.id);
+
+    // Use an identical cloneScope in the parent as the child to have identical
+    // behavior for proxied vs direct calls. If all APIs are proxied, then the
+    // parent cloneScope does not really matter (because when the message
+    // arrives locally, the object is cloned into the local clone scope).
+    // If all calls are direct, then the parent cloneScope does matter, because
+    // the objects are not cloned again.
+    Object.defineProperty(proxyContext, "cloneScope", {
+      get: () => this.cloneScope,
+    });
+
     // Many APIs rely on this, so temporarily add it to keep the commit small.
     proxyContext.setContentWindow(this.context.contentWindow);
 
     // Synchronously unload the ProxyContext because we synchronously create it.
     this.context.callOnClose({close: proxyContext.unload.bind(proxyContext)});
   }
 
   getFallbackImplementation(namespace, name) {