Thanks a lot for your help newseed, its almost resolved, but in firefox I don't know why he gives a margin on the top of the grey box inside the content, do you know why? I don't have any margin on top.
I still need to make the scroll overlap the content, I'm trying to use z-index but no success, any ideas for this? because when I have scroll on content its no problem, but when not the grey box has a margin on the bottom where the scroll was suppose to be.