have you just updated from a 3.6 version of firefox? you can bring up the menu bar with the tools menu by pressing the alt key once or you can access the options via the new orange firefox button on the top left of the browser window.

Or if, like me, you want to see the menu bar at all times, right click next to the + right of the current window tab and select the "menu bar" (name might differ since i'm using a local language version)