I have a dual monitor setup. I want to use other applications (on my other monitor) whilst watching fullscreen YouTube videos. However, when I do anything, such as click my mouse, the video returns from full screen. Are there any settings I can configure, or an addon I could install, that could fix this problem?

This is not a problem on Google Chrome, and I want to continue using Firefox, but if I can't fix this problem then I'd have to switch browsers as this is important to me.