When Java applet invokes JavaScript which executes window.showModalDialog, get popup with about:blank

When using a Java applet that invokes a Javascript function that in turn calls window.showModalDialog, a pop-up dialog that only contains "about:blank" results. Selecting "show desktop" from the Windows 7 taskbar and then viewing the FireFox window results in the true dialog content being displayed. Everything worked properly until very recently (and old versions of FireFox work) and I suspect something was changed in FireFox as a result of the recent Java security failures.