In the site in the URL, and in the testcase-to-be-attached, the body sizing behavior differs between WebKit and Firefox 3. The body has a single, absolutely-positioned child which has z-index: -10. In Firefox, the body has zero height, and the child gets events. The page is loaded in quirks mode (according to the Info window). In WebKit, the body covers the viewport, and therefore obscures the child, which does not get events.
Created attachment 26621 [details] Testcase
Per Hyatt this is an IE quirk that firefox doesn't respect.
Sounds like a WONTFIX then.