May seem silly but do you use a popup preventer (NoAds a very popular preventer is actually a window preventer if you check the window list or popup list you should find Mozilla there). It is also possible to ad Mozilla to your popup preventer in Mozilla itself. If you get a popup that is lets say a porn site and you right click and add to popup list and the popup window name just says Mozilla build ,xxxx you will not be able to use Mozilla it will supress its own window.
