Yes, this can happen when not browsing, perhaps while I am running another application. There does not appear to be any rhyme or reason to the disappearance. Glad to see that someone else has seen the problem. I am not alone. BTW, I have not seen this happen in my Mint installation of FF, only the Ubuntu. (Just to be clear, I am running Gnome Classic and not Unity. I find Unity somewhat off-putting, annoying really).