Do you have a status bar at the bottom of the Firefox screen? If that is where the clock was it would have come from an extension. You should look through your extensions list to check; if you find an appropriate one check the options. The Status Bar may have been set to autohide - mouse over the bottom of the screen. From Fx4 0n the Status Bar has been eliminated by default. There is a clock on the Windows Task Bar which may have also been set to autohide. Let us know how you get on.