I've only just now encountered this problem, and it's making me crazy. I didn't know about the CTRL+W shortcut; I'll use that now, but meanwhile, why has the X disappeared? I have my tab bar set to "always open," with the bar narrowed to the point where there's some text plus the identifiable icon (If I widen it any more, the icon changes to something that is FAR less identifiable), and there is NO X to close the page.
OK: Just to test what I was saying, I attempted once again to widen the tab bar a little bit more, and I managed to get the X to appear, but I am not confident that it'll still be there when I reopen this. Maybe others will have had the same experience.