Firefox asks for a general password using a drop-down window when opening a page with a log-on section, presumably for an autofill. (It's annoying and I wish I could get rid of it, but that's another story.) My home page has a log-on section, so the prompt appears. But, when I try to choose a bookmarked page, and if the prompt appears while I'm doing it, the bookmark menu covers the prompt. Nothing I do dispels the bookmark menu, whether clicking away from the menu, choosing the menu item, or even just typing the password. I have to restart, sometimes requiring 'force quit' from the Activity Monitor. I automatically clear my history when I quit. Seems obscure enought that it might be a bug...