fix failing FF tests

[bantic]

Firefox changed its behavior when focusing a content editable element in version 46, and this is causing the SauceLabs tests to fail. When we render the editor in a browser window that is not active, if options.autofocus is true, the editor focuses its element. In Chrome, Safari, and previous versions of Firefox this makes the element the document.activeElement and also sets the Selection at the end of its content. In newer Firefox window.getSelection === document.body, rather than the expected text node at the end of the editor's element. This causes a number of tests to fail at SauceLabs on Firefox because it does not appear to be the frontmost/active window while tests are running.