Tweak Firefox's "Responsiveness" Config Setting

When Firefox is loading a page it uses one of two modes: There's a high priority mode that doesn't pay as much attention to your mouse and keyboard, but loads the page faster. There's also a lower priority mode that interrupts the parser more often to respond to input events. It's even documented at Mozillazine.

What we can do is tweak the amount of time before Firefox switches from lower priority mode back into high priority mode.