Turning Off "Force Shutdown"?

I have a Dell Desktop running Windows 7, 64 bit. I usually run Mozilla Firefox each time I use my computer. When I shut down my computer, I do so without closing Firefox because I want to be able to quickly re-access the open browsers when I later restart my computer and Mozilla Firefox. The problem is that a screen appears asking if I want to force a shutdown because some Firefox process is still running. So the computer will not shut down unless I indicate that I want to force a shutdown and then a second click is required to confirm this. If I don't do this two step process, the computer will remain on indefinitely. This is pretty annoying. I have a laptop which also runs Window 7, but for some reason, the force shutdown issue does not arise - the computer just shuts down even when I leave Mozilla Firefox open when shutting off the laptop.Does anyone know how to change this so that the computer will automatically force the shutdown without me having to do the 2 step process to get it to do so?