Problem with Firefox/Java

This is a little bit weird, but the pop-up dialog windows of Firefox are not being displayed properly anymore. Nor are the windows in the Java apps ran from the Firefox. Below is a screenshot.

Any ideas what the cause might be?

PS: Haven't got Photoshop installed on this machine, thus excuse the stupid border around the box left by paint.

You aren't using a new skin or anything? Have you tried rebooting xD

Sorry I can't offer any better ideas.

I am using a visual theme under Windows, but that shouldn't be a problem. The Java for instance works fine in IE7, but totally refuses do display buttons in FF2.=/

Thanks nevertheless.