If you'd just like to start using Sage, head to the website to get started.
$ git clone firstname.lastname@example.org:petea/sage.git $ cd sage $ curl [Gecko SDK download URL] | tar -x $ ant
If all goes well, you'll find the XPI file under the sage/build folder.
With the XPI built, you can install it by opening it from Firefox or copying it to your Firefox profile directory. Either way, you'll need to restart Firefox.