I would like to know if it is possible to block these type of windows that appear when you scroll to the bottom of the web page or even when you just load them. It seems to me they are CSS. Generally they try to force you to subscribe to a newsletter or register to the website.
I use Firefox 36
@Dean-meehan
mentioned a standard way of getting rid of such things, I suppose by blocking you meant permanently. Therefore, I suggest a very easy approach which is an addon Nuke Anything Advanced here. It gives you two options : 1) Temporary, 2) Permanent. I prefer permanent when I know I'll come back later again to this site.Remove Object
. Done.