>>It seems to be some hide script on your navigator, try to open the levelextreme.com in incognito mode to prevent the issue and/or using other navigator for a while to see if the problem appears again. This will let you know what is going on.
>Are you leaving opened the site for days and days? maybe that is also the problem, I never kept opened this site more than 2 consecutive days.

>I never close it. For years. It's a problem only recently. And my browser is Waterfox, where NoScript simply refuses to work.

Hi Dragan, first time I see this Waterfox, I will give it a try. I have no other idea about your problem. Oh wait: "It must be a firewall issue" (http://www.developerexcuses.com/) :)