User Agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:40.0) Gecko/20100101 Firefox/40.0 Build ID: 20150521004003 Steps to reproduce: Enable E10S Make an SDK addon which has require("sdk/ui"). You don't actually have to use the UI module, just require it. Open the browser console and navigate to any web page. Actual results: The following is logged to the browser console (reproduced in Nightly and Developer Edition): TypeError: target.QueryInterface is not a function Stack trace: getFrameElement@resource://gre/modules/commonjs/toolkit/loader.js -> resource://gre/modules/commonjs/sdk/window/utils.js:443:4 getFrame@resource://gre/modules/commonjs/toolkit/loader.js -> resource://gre/modules/commonjs/sdk/input/frame.js:32:39 lift/</<@resource://gre/modules/commonjs/toolkit/loader.js -> resource://gre/modules/commonjs/sdk/event/utils.js:163:28 emitOnObject@resource://gre/modules/commonjs/toolkit/loader.js -> resource://gre/modules/commonjs/sdk/event/core.js:112:9 emit@resource://gre/modules/commonjs/toolkit/loader.js -> resource://gre/modules/commonjs/sdk/event/core.js:89:38 receive@resource://gre/modules/commonjs/toolkit/loader.js -> resource://gre/modules/commonjs/sdk/event/utils.js:115:5 InputPort.prototype.observe@resource://gre/modules/commonjs/toolkit/loader.js -> resource://gre/modules/commonjs/sdk/input/system.js:100:5 ObserverParent.notify@resource://gre/modules/RemoteAddonsParent.jsm:354:9 ObserverParent.receiveMessage@resource://gre/modules/RemoteAddonsParent.jsm:344:9 In addition, when the browser is first run (rather than when loading a page), "JavaScript error: resource://gre/modules/AddonManager.jsm, line 20: Error: You cannot use the AddonManager in child processes!" is logged to the console, and I think it's the same issue. The issue is only present with E10S enabled. Expected results: No error should be logged.

I was able to reproduce it with Nightly 44.0a1 (2015-10-12) and Aurora 43.0a2 (2015-10-08). Steps: - jpm init - Added "var ui = require('sdk/ui');" to index.js - jpm run -b "C:\Program Files (x86)\Firefox Developer Edition\firefox.exe"

I can't reproduce this on Nightly nor in the following combination of "SO / Firefox version": - Linux: 44.0a2, 43.0, 43.0a2 43.0b2 43.0b9 - Windows 8: 44.0a2, 43.0 Feel free to reopen if you are still able to reproduce it on Nightly or on recent Firefox 43 or Firefox 44 builds.