Youtube videos wont go to an actual full screen.

i have no clue what happened, just the other day when i clicked on full screen on youtube videos, it actually went to full screen. now when i click on full screen it just fills up my firefox with the video besides the top address bar. i took a screen shot of a random video if that helps. i would like it to go back to the way it used to be, and the way other videos play still.