Steps to reproduce: 1) Build with assertions enabled on Mac 2) Run TEST_PATH=toolkit/components/startup/tests/browser/browser_bug537449.js make -C obj-ff mochitest-browser-chrome Actual results: When running this test on Mac, I see ###!!! ASSERTION: Oops! You're asking for a weak reference to an object that doesn't support that.: 'factoryPtr', file nsWeakReference.cpp, line 109 ###!!! ASSERTION: Uh, IsInModalState() called w/o a reachable top window?: 'Error', file /Users/Shared/Projects/mozilla-html5/dom/base/nsGlobalWindow.cpp, line 5907 Expected results: No assertions.