Modifying dom.disable_window_open_feature.scrollbars probably won't help for such pages. These dom.disable_window_open_feature prefs are merely about pop-up windows opened via a window.open() function call. If you do not see a horizontal scroll bar then websites might be suppressing them. A possible workaround would be to zoom the page out if only a small part is missing. Can you post a link to a publicly accessible page (i.e. no authentication or signing on required)?