Firefox is not anymore singleprocess. It will fall back to singleprocess, if you have an extension installed that's not yet compatible with multiprocess. With the aforementioned Firefox 57, they will switch over to a new extension API and this will also result in those old singleprocess extensions being thrown out, so that's why they haven't bothered trying to make this transparent to users.

Also, they are switching out the UI with Firefox 57, so I hope you like this one, too: http://design.firefox.com/people/shorlander/