User Agent: Mozilla/5.0 (Windows NT 6.3; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/45.0.2454.85 Safari/537.36 Steps to reproduce: (pure FF v40.0.3 (No user; no sync etc.); windows 8.1) 1) Open new private window (single tab only) 2) Maximize window (important!)(f.e. win + arrow up) 2) Start video (youtube etc.) 3) Enter video full screen mode (bottom right video player corner; split rectangle) 4) Close window ctrl + w or alt + F4 5) Open new private window again Actual results: Firefox enter full screen mode (equal of clicking F11) from very begging Won't happen if browser was not maximized - step 2 Expected results: Firefox should start in normal mode - not in full screen like it happens when browsing web in not maximized window (If it was feature to help user be more win 8 like, there should be settings toggle to allow users disabling it)

This seems like a duplicate of 1137009. Will it reproduce on nightly?

You are right. This is duplicate of 1137009 (I'm on 40.0.3 and milestone of 1137009 is mozilla44).