Mouse Lock API Points of Interest
This page is for tracking different parts of the Firefox code that relate to the mouse in different ways.
I suggest looking at widget/src/*/nsWindow.cpp for starters. It seems like the OS-Specific mouse stuff is all done in there.
Try researching in different OS APIs about setting the mouse cursor position directly, it should turn up some interesting results.
Here's some code that actually locks the mouse into a rectangle with no width or height in the center of the screen in WIN32.