I am now in another computer, PowerBook, I noticed the same thing. Usually I don't have any Mozilla products installed in my PowerBook, but in order to check Netscape's secnews, I decided to install Mozilla 1.7 (for OS X of course). Again, with the Exposé again (F9), I noticed the hidden window again.
(Detail of the screenshot: Quit Safari, Quit Mozilla 1.7, and re-launch it. then pressed F9. Screenshot taken by Snapz Pro X (licensed copy). http://sillydog.org/ is the default home page of my Mozilla 1.7, and I haven't surfed to any other pages.)
From the above screenshot, the smaller window labelled "hidden" would only appear under Exposé. It is well hidden under normal usage. What is the hidden window for?
I checked Safari browser, no hidden windows at all. I will check if this exist in Firefox when I back to my PowerMac G4.
UserAgent: Mozilla/5.0 (Macintosh; U; PPC Mac OS X; en) AppleWebKit/125.2 (KHTML, like Gecko) Safari/125.8




3.0 
