--- a/testing/marionette/driver.js
+++ b/testing/marionette/driver.js
@@ -146,16 +146,17 @@ this.GeckoDriver = function(appName, dev
 
     // Selenium 2 compat
     "platform": Services.sysinfo.getProperty("name").toUpperCase(),
 
     // proprietary extensions
     "XULappId" : Services.appinfo.ID,
     "appBuildId" : Services.appinfo.appBuildID,
     "device": device,
+    "processId" : Services.appinfo.processID,
     "version": Services.appinfo.version,
   };
 
   this.mm = globalMessageManager;
   this.listener = proxy.toListener(() => this.mm, this.sendAsync.bind(this));
 
   // always keep weak reference to current dialogue
   this.dialog = null;