Michael,
I use Firefox too and I've had trouble with windows "vanishing" too, but they don't really disappear, but they just go behind another window.

I've taken a screen shot of the operation on Firefox:
http://www.flickr.com/photos/diffuse/4839024950/

Do you have a task bar at the bottom of your screen like the one in that image? (It begins with "Start").

In my example, you can also see two buttons, "BS: Mudcat~~Why..." and "Link Maker - Mozilla Fi". Those mean that there are two windows open. When I can see one of those windows on my monitor, I can click the button for the other and that window will come to the front.

rags