All the browser chrome including the tab switcher, awesomebar, firefox menu, and toolbars, but not the standard minimize/unmaximize/close buttons, will sometimes turn invisible while the window is inactive. Clicking back into the window restores them. I have a dual monitor setup, and firefox is maximized on one monitor while I'm working on other applications in the other. This is an intermittent problem, and has been happening for about a week now, starting from a fresh install of Firefox with a clean profile.