Page is left visible when user is logged out in firefox

In chrome when a user is timed out and logged out they are taken to a logged out screen.

In firefox however they stay on the same screen. They get an access denied message if they refresh the page, but the logged in page is visible until they do this.

This is an issue for me as my site has sensitive information. If a user was to leave a page open and leave there desk, I want them to be logged out and for the screen to not be visible.

Thanks