Can't make firefox window to fullscreen.

''duplicate of https://support.mozilla.org/en-US/questions/1096087'' Hello, I am using firefox in my laptop running windows 7. There I cannot make it to fullscreen mode. When I try to do that, it looks like the attached image. That strange thing comes in up-right corner, covering the controls, and blocks the whole window going fullscreen. The problem disappears after an update or changing the theme or reinstalling firefox completely. But it starts again after few days. Feeling irritated. Subhajit Paul.