Installation is very simple. You can just find your way to the extension in your firefox extension gallery and install it. Otherwise download it from the gspace website which is very simple and described here.
With your firefox browser go to GSpace website. Go to their download page. And select your operating system. Confirm your installation and you're done and free to use the Gmail free memory as your personal stash for storing files. If your firefox fails to recognize your firefox plugin then just go to the downloaded file and open with firefox web browser. That should do.