The non-standard window.content object is no longer available from web content on Firefox Nightly 57 and later. It will be removed from other channels as well once Telemetry proves low usage.

This object isn’t useful because it’s just an alias of window.top or simply window in many cases, but it may have been misused for browser detection purposes on the web, like window.controllers that was once removed with Firefox 29 but added back to retain backward compatibility. Web developers should always avoid using these kind of non-standard features.