Have you ever noticed that sometimes Firefox isn't very responsive while loading a page? As a frequent StumbleUpon user, this behavior grates on my nerves so I went looking for a solution.
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.