These are dymanic "<div>" which are called through javascript and are not considered browser popups.
The html code in tha page has them visible on-load and hidden on-demand.
They are not new browser windows, it's just some code executed within the page, something like this forum does when you click to enlarge an image.
The only way to disable it is to disable javascript I guess , but many sites use javascript to execute usefull functions and I don't think it would be a good idea. You can disable it though, if you're browsing these sites extensively, and re-enable it when you close them.
Cheers