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.
WIN32 API
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.
- http://pastebin.com/gNmh1uhz
- [1](requires adding the mMousePos and mMouseLock member variables)