This is a bug in Firefox 3 where it launches into full screen mode and completely covers the top and bottom GNOME panels as well as its own title bar. While in this buggy mode Firefox cannot be resized, and can only be closed from within the menu.
File -> Quit
A temporary fix involves pressing [...]

Use the Keyboard Shortcuts tool in GNOME to customize the default keyboard shortcuts for your system. If you have a laptop that has special media buttons you can configure them here as well. In some distributions such as Ubuntu these media buttons may be automatically configured but you may still need to fine tune them [...]