Having a clock in the toolbar of fullscreen Firefox will make fullscreen more useful as seeing the current time is one of the essential uses of the system toolbar. When users enter fullscreen Firefox, we should do what we can to make the experience seamless and reduce the need to exit or show the system toolbar.
Created attachment 8624033 [details] [diff] [review] Simple patch This is a simple patch that adds a clock widget to the tabs toolbar when the browser is in fullscreen or customization mode (the latter to allow removal). It is hidden when the browser is not in fullscreen mode, regardless of where it is placed. The tooltip shows the current date. Clicking on the button toggles between showing the time in 12-hour and 24-hour format. The default formatting is locale-dependent.
Short video showing it working, http://screencast.com/t/aFWJOqmOareO