Firefox itself does not have native features for this. However, you can download and install the PowerMenu application for Windows at [http://www.abstractpath.com/powermenu/]. It installs very quickly. When you want to watch your video, put it into its own Firefox window (you can do this by right-clicking the video tab and selecting "Move to new window". In this new window, right-click any one of the three Windows buttons at the top right corner (Minimize, Resize, Close). Click "Always On Top" from the menu. This window will now remain on top of all other windows until you uncheck "Always On Top". I hope that helps!