Install git

There is a nice git installer. Download it from here, and run the installer.

Get NXT Improved Firmware sources

Now that git is installed, it is easy to get the sources.

By accessing NXT Improved Firmware, you accept the license.

Now open a terminal and type:

cd $HOME/Documents
git clone http://git.ni.fr.eu.org/nxt-firmware.git/

That's it! You can now browse source code.