fix #452 MouseEvent.pageX/Y not supported in Firefox 3.6 (FireDomEvent)

Since e4e3296618f13aa307a7acc4e9a452381f003985, multiple test cases
were failing in Firefox 3.6 due to the bug in the browser where
an error was thrown when trying to override pageX/pageY of a custom
event. This commit adds a workaround for that bug.