For details of how to restore it, see https://support.mozilla.com/kb/menu+bar+is+missing

Thanks. Why do they do that without forewarning??????

Mozilla have never hid the menus on Firefox 3.6.*

It may have happened by accidentally clicking on something, for example if you right-clicked on a toolbar it is easy to hide the menu bar.

Please click the Solved button next to the answer that solved your Firefox support issue, when you are logged in.