Hidden Window in FireFox, Mozilla, Netscape, Thunderbird

Hi!  I'm trying to find an answer to this issue regarding the infamous hidden 
window in almost every Mozilla product.

(For those who don't know, open up [insert browser here] and use Expose to show 
all windows - You'll find a small window that you've never asked for, and can't 
control)

Firefox exhibits this behaviour, as do Mozilla, Netscape, and Thunderbird (Of 
all things!).  Camino is the only Mac product that doesn't seem to do this.

Any speculation as to what the window is for or why it's there would be 
welcomed, but *real answers* would be preferred.