Image in negative appearing below windows

[andreseso]

Hello,

Right now I have on one of my monitors a black area showing up when I put one of my windows over it. It is the area for showing a youtube video. I have no youtube open on any tab. This black area does not show up with the Take Screenshot app nor with Shutter.

I would like to make everything completely opaque so nothing shows up below any of my windows. I have Visual Effects set to High but I have not been able to find anywhere where I can configure transparency nor visual effects.nor in System Preference nor in any of my menus.

I am using Maverick.

Love

[andreseso]

I am convinced it is a naughty firefox extension that makes the image that is not captured by shutter appear under my windows and that shows up on the white areas of my windows in a specific point.

I attach a screenshot of my firefox addons in case any of you know which addon I have to disable.

Love