sizemode - Archive of obsolete content
listen to the sizemodechange event dispatched to the dom window to get notified when the window state changes.

XUL Events - Archive of obsolete content
sizemodechange sent when a window is minimized or un-minimized, switched between full screen and windowed mode, or maximized.

window - Archive of obsolete content
listen to the sizemodechange event dispatched to the dom window to get notified when the window state changes.

Event reference
sizemodechange addons specific window has entered/left fullscreen mode, or has been minimized/unminimized.