Is this on your Windows XP system or on a different version of Windows?

The most common reason for display glitches is an incompatibility between Firefox and your graphics card/graphics chipset driver software. The standard workaround is to disable Firefox from using hardware acceleration. You can do that here:

"3-bar" menu button (or Tools menu) > Options > Advanced

On the "General" mini-tab, uncheck the box for "Use hardware acceleration when available"

This takes effect the next time you exit Firefox and start it up again. Any difference?

In other cases, users appeared to be helped by choosing a different Windows theme, but I'm not sure this is relevant for Windows XP (pre "Aero").

By the way, as workarounds for the blacked-over buttons:

• You can double-click just to the left of the buttons to switch between a maximized and resizable window
• You can press Alt+Spacebar to call up the window control menu with equivalents for the buttons
• Some users reported they can click "through" the black covering and use the buttons that way (of course, you need to have good aim)