On two customers machines with Windows 7 and Firefox 26 happens following: At the webpage I have a div container which display the several different content, depending on the menu item choosed. Only with 2 contents there is a wrong display: Below the container there is a single line of content text which should not be visible (outside of container). This line disappears after some seconds. I can't reproduce this on Mac, on Windows XP and Windows7, where the Windows machines are running in VMware. Neither a friend of mine can reproduce this. On the clients machine the wrong display is not shown on IE. This machine has a ATI Mobility Radeon HD3400 graphics card. You can test this behavior at this 2 urls: http://www.sterac.com/test/de/italien_griechenland_transport.php http://www.sterac.com/test/de/uebersee_transport.php You can see the wrong line at this screenshot: http://gondomir.de/daten/steracfehler.jpg On all other menu items the wrong display doesn't appear. The page is valid XHTML 1.0 strict. I also can't see a significant difference between the pages which works and which not. The pages makes heavy use of JavaScript.