I try to close a tab whether using the mouse or CntrlF4 or CntrlW The actual tab doesn't disappear, or does the page. However if I then navigate to adjacent tab (mouse or CntrlTab) and then back again, I find that I cannot select the tab that i tried to close. Similarlr if I press the "List all tabs" at the extreme right of the tabs bar, the page that I close isn't there. But The tab for the closed page is still present in the tabs bar. It doesnt seem to be a property of the actual page, a google results page, indeed a mozilla support page, youtube, whatever.