popups not closing in older FF versions

Steps to Reproduce

run attachment open various popups a try to close them by clicking outside the popup
example contains:popup, menupopup, chosenbox-popup

Actual Result

popups don't close when clicking the background

Expected Result

popups close when clicking outside of them

Debug Info

the onFloatUp event is not fired in FF38 - not closing any popups

Root Cause

side effect of ZK-3193 (https://github.com/zkoss/zk/commit/a870058daead9423407f0097c2de8fb77d8c73fb#diff-0968c05a9ac2112ca4a43ee9785735f3R600)

it uses an experimental API (MouseEvent.offsetX/Y) only available in FF39+
it should be avoided to depend on experimental API
https://developer.mozilla.org/en-US/docs/Web/API/MouseEvent/offsetX
https://developer.mozilla.org/en-US/docs/Web/API/MouseEvent/offsetY