"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."

With Firefox, you can have a 'Tab Bar' up the top (or bottom) that shows you all the many windows you can have open at once - I won't say WHERE you go to turn that on or off, as I have so many other plug ins that may be modifying that :-) but you CAN also have it set up so that each new url opens in a new window - Ragdalls pic shows that. Personally I find the multi tabs easier.