Sometime recently the History > Recently Closed Tabs function is slightly broken, and it appears due to the loading of website icons. See Screenshot 1, where "Recently Closed Tabs" has loaded the ''list'' of sites but has not loaded the website icons. If you now move the mouse pointer into the sub-window containing the list of sites, that list will disappear and you have to click "Recently Closed Tabs" again. If however you wait for Firefox to load all the website icons, as shown in Screenshot 2, you can move the mouse pointer into the website list window without the list window disappearing. This is an annoyance in slower connections, e.g. public hotspots or while tethering in poor coverage locations, as the icons may take some time to load.