The Object.keys of Firefox has bug

The Object.keys of Firefox has bug, you can try like this in Firefox:

	<script>
        console.log(Object.keys(window));
        if(top == window){}
        console.log(Object.keys(window));
	</script>

Results here:

    ["window", "document", "InstallTrigger", "console", "process", "global", "Mocha", "mocha", "sinon", "expect", "location", "mouseEvents", "msPointerEvents", "keyEvents", "uiEvents", "bubbleEvents", "touchEvents", "gestureEvents", "before", "after", "beforeEach", "afterEach", "context", "describe", "xcontext", "xdescribe", "specify", "it", "xspecify", "xit", "getInterface"]
    ["window", "document", "InstallTrigger", "console", "process", "global", "Mocha", "mocha", "sinon", "expect", "location", "mouseEvents", "msPointerEvents", "keyEvents", "uiEvents", "bubbleEvents", "touchEvents", "gestureEvents", "before", "after", "beforeEach", "afterEach", "context", "describe", "xcontext", "xdescribe", "specify", "it", "xspecify", "xit", "getInterface", "top"]

Some variables can't be read by Object.keys, until the variable been used in code.