Closing ModalWindow makes some form fields unclickable

After closing a ModalWindow, sometimes (1 out of 5-10) some form fields (text, select) will become unclickable/read-only (but not grayed out), as if the ModalWindow weren't closed. Those form fields aren't necessarily those covered by the ModalWindow.

It works fine in IE.

In the error console in Firefox, there are a few errors (the error message has been translated from Chinese):

Warning: error parsing the value for "height". Call interrupted
Source: http://localhost:8888/?wicket:interface=:12::::
Line: 0

Warning: error parsing the value for "filter". Call interrupted
Source: http://localhost:8888/resources/org.apache.wicket.extensions.ajax.markup.html.modal.ModalWindow/res/modal.css
Line: 26

Warning: Unknown property "-moz-opacity". Call interrupted
Source: http://localhost:8888/resources/org.apache.wicket.extensions.ajax.markup.html.modal.ModalWindow/res/modal.css
Line: 24