Currently we build a pretty big ThebesLayer for the standard chrome window: the chrome at the top and the status bar at the bottom mean the visible area's bounds are huge. (In fact display list construction for the chrome window doesn't know about the content area so the visible area is the entire window, but bug 130078 will fix that.) This large ThebesLayer takes memory and time to initialize, but is mostly unused. The status bar may be off by default for FF4, but some users will probably reenable it. Assuming bug 130078 lands, an expedient way to fix this would be to force the status bar into its own layer. A custom XUL attribute would suffice.