This addon inserts X-Forwarded-For into the HTTP Request header. The value is currently the IP 8.8.8.8 witch is an IP in United states. This will make some websites think that you are a proxy that is forwarding a client from United States.
To build the addon you have to use the addon builder sdk from mozilla. Download it here: https://ftp.mozilla.org/pub/mozilla.org/labs/jetpack/jetpack-sdk-latest.zip
After you setup the addon-sdk start firefox with the addon installed automaticly with the following command:
cfx run
If you want to build an xpi package run:
cfx xpi