--- a/toolkit/components/extensions/NativeMessaging.jsm
+++ b/toolkit/components/extensions/NativeMessaging.jsm
@@ -1,15 +1,16 @@
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 "use strict";
 
 this.EXPORTED_SYMBOLS = ["HostManifestManager", "NativeApp"];
+/* globals NativeApp */
 
 const {classes: Cc, interfaces: Ci, utils: Cu} = Components;
 
 Cu.import("resource://gre/modules/XPCOMUtils.jsm");
 
 const {EventEmitter} = Cu.import("resource://devtools/shared/event-emitter.js", {});
 
 XPCOMUtils.defineLazyModuleGetter(this, "AppConstants",
@@ -166,27 +167,22 @@ this.NativeApp = class extends EventEmit
     super();
 
     this.context = context;
     this.name = application;
 
     // We want a close() notification when the window is destroyed.
     this.context.callOnClose(this);
 
-    this.encoder = new TextEncoder();
     this.proc = null;
     this.readPromise = null;
     this.sendQueue = [];
     this.writePromise = null;
     this.sentDisconnect = false;
 
-    // Grab these once at startup
-    XPCOMUtils.defineLazyPreferenceGetter(this, "maxRead", PREF_MAX_READ, MAX_READ);
-    XPCOMUtils.defineLazyPreferenceGetter(this, "maxWrite", PREF_MAX_WRITE, MAX_WRITE);
-
     this.startupPromise = HostManifestManager.lookupApplication(application, context)
       .then(hostInfo => {
         if (!hostInfo) {
           throw new Error(`No such native application ${application}`);
         }
 
         if (!hostInfo.manifest.allowed_extensions.includes(extension.id)) {
           throw new Error(`This extension does not have permission to use native application ${application}`);
@@ -215,32 +211,46 @@ this.NativeApp = class extends EventEmit
         this._startStderrRead();
       }).catch(err => {
         this.startupPromise = null;
         Cu.reportError(err instanceof Error ? err : err.message);
         this._cleanup(err);
       });
   }
 
+  /**
+   * @param {BaseContext} context The scope from where `message` originates.
+   * @param {*} message A message from the extension, meant for a native app.
+   * @returns {ArrayBuffer} An ArrayBuffer that can be sent to the native app.
+   */
+  static encodeMessage(context, message) {
+    message = context.jsonStringify(message);
+    let buffer = new TextEncoder().encode(message).buffer;
+    if (buffer.byteLength > NativeApp.maxWrite) {
+      throw new context.cloneScope.Error("Write too big");
+    }
+    return buffer;
+  }
+
   // A port is definitely "alive" if this.proc is non-null.  But we have
   // to provide a live port object immediately when connecting so we also
   // need to consider a port alive if proc is null but the startupPromise
   // is still pending.
   get _isDisconnected() {
     return (!this.proc && !this.startupPromise);
   }
 
   _startRead() {
     if (this.readPromise) {
       throw new Error("Entered _startRead() while readPromise is non-null");
     }
     this.readPromise = this.proc.stdout.readUint32()
       .then(len => {
-        if (len > this.maxRead) {
-          throw new Error(`Native application tried to send a message of ${len} bytes, which exceeds the limit of ${this.maxRead} bytes.`);
+        if (len > NativeApp.maxRead) {
+          throw new Error(`Native application tried to send a message of ${len} bytes, which exceeds the limit of ${NativeApp.maxRead} bytes.`);
         }
         return this.proc.stdout.readJSON(len);
       }).then(msg => {
         this.emit("message", msg);
         this.readPromise = null;
         this._startRead();
       }).catch(err => {
         if (err.errorCode != Subprocess.ERROR_END_OF_FILE) {
@@ -299,26 +309,25 @@ this.NativeApp = class extends EventEmit
       }
     });
   }
 
   send(msg) {
     if (this._isDisconnected) {
       throw new this.context.cloneScope.Error("Attempt to postMessage on disconnected port");
     }
+    if (Cu.getClassName(msg, true) != "ArrayBuffer") {
+      // This error cannot be triggered by extensions; it indicates an error in
+      // our implementation.
+      throw new Error("The message to the native messaging host is not an ArrayBuffer");
+    }
 
-    let json;
-    try {
-      json = this.context.jsonStringify(msg);
-    } catch (err) {
-      throw new this.context.cloneScope.Error(err.message);
-    }
-    let buffer = this.encoder.encode(json).buffer;
+    let buffer = msg;
 
-    if (buffer.byteLength > this.maxWrite) {
+    if (buffer.byteLength > NativeApp.maxWrite) {
       throw new this.context.cloneScope.Error("Write too big");
     }
 
     this.sendQueue.push(buffer);
     if (!this.startupPromise && !this.writePromise) {
       this._startWrite();
     }
   }
@@ -383,16 +392,17 @@ this.NativeApp = class extends EventEmit
       disconnect: () => {
         if (this._isDisconnected) {
           throw new this.context.cloneScope.Error("Attempt to disconnect an already disconnected port");
         }
         this._cleanup();
       },
 
       postMessage: msg => {
+        msg = NativeApp.encodeMessage(this.context, msg);
         this.send(msg);
       },
 
       onDisconnect: new ExtensionUtils.SingletonEventManager(this.context, "native.onDisconnect", fire => {
         let listener = what => {
           this.context.runSafeWithoutClone(fire, port);
         };
         this.on("disconnect", listener);
@@ -437,8 +447,11 @@ this.NativeApp = class extends EventEmit
       responsePromise.catch(() => {});
 
       this._cleanup();
     });
 
     return result;
   }
 };
+
+XPCOMUtils.defineLazyPreferenceGetter(NativeApp, "maxRead", PREF_MAX_READ, MAX_READ);
+XPCOMUtils.defineLazyPreferenceGetter(NativeApp, "maxWrite", PREF_MAX_WRITE, MAX_WRITE);

--- a/toolkit/components/extensions/ext-runtime.js
+++ b/toolkit/components/extensions/ext-runtime.js
@@ -61,16 +61,17 @@ extensions.registerSchemaAPI("runtime", 
 
       connectNative(application) {
         let app = new NativeApp(extension, context, application);
         return app.portAPI();
       },
 
       sendNativeMessage(application, message) {
         let app = new NativeApp(extension, context, application);
+        message = NativeApp.encodeMessage(context, message);
         return app.sendMessage(message);
       },
 
       get lastError() {
         // TODO(robwu): Figure out how to make sure that errors in the parent
         // process are propagated to the child process.
         // lastError should not be accessed from the parent.
         return context.lastError;

--- a/toolkit/components/extensions/test/xpcshell/test_native_messaging.js
+++ b/toolkit/components/extensions/test/xpcshell/test_native_messaging.js
@@ -264,17 +264,18 @@ while True:
     let listener = (what, msg) => {
       equal(msg, MSG, "Received test message");
       app.off("message", listener);
       resolve();
     };
     app.on("message", listener);
   });
 
-  app.send(MSG);
+  let buffer = NativeApp.encodeMessage(context, MSG);
+  app.send(buffer);
   yield recvPromise;
 
   app._cleanup();
 
   do_print("waiting for async shutdown");
   Services.prefs.setBoolPref("toolkit.asyncshutdown.testing", true);
   AsyncShutdown.profileBeforeChange._trigger();
   Services.prefs.clearUserPref("toolkit.asyncshutdown.testing");