You need to have Chromeless downloaded in order to run Despin. You can get it by typing the following command:
git clone git://github.com/mozilla/chromeless.git
In order to download Despin you have to clone the repository and fetch the submodules:
git clone git://github.com/past/despin.git git submodule update --init --recursive
If you downloaded chromeless and despin in sibling directories, you can run despin by typing the folowing command in the despin directory:
Otherwise you will have to substitute the full path to the chromeless script in your system in the command above.
Despin is licensed under the standard Mozilla MPL/GPL/LGPL tri-license, same as Firefox.