According to the Help File, if "Full screen" in Video config is checked, FCEUX will enter full screen when it is loaded. But this doesn't take effect, FCEUX still in window mode when it runs again, and the "Full Screen" item is not checked.
Please make the menu bar usable when in full screen, because if some of the operation in the menu is not assigned a shortcut key, it would be impossible to do it in full screen, you have to reverse to window and perform it. And if the Toggle Fullscreen has no shortcut, you even can't reverse to window unless you exit the emulator by pressing Alt+F4!
Despite to the problem above, I suggest to make the menu in fullscreen appears when mouse cursor moves to the top of screen, like some of the other softwares, and you can press Alt+? to call the menu.