Cannot close an error box after multiple error boxes created in firefox

steps to reproduce

1. run the attached zul in Firefox
2. focus on the first intbox
3. press tab to switch to 2nd intbox
4. repeat above 2 steps quickly for several times

actual result

ZK create multiple error boxes, and you cannot close some of them.

Workaround

• use constraint message like constraint="no positive:error message"

  debug info

• happen in firefox (not in Chrome and IE9)