Gspace is an extension that allows you to use your Gmail Space (4.1 GB and growing) for file storage. It acts as an online drive, so you can upload files from your hard drive and access them from every Internet capable system. The interface will make your Gmail account look like a FTP host.
After the installation, you’ll get an option called “Gspace” in your “Tools” menu, which opens the GSpace window.

It’s great for storing/sharing files with your friends. Also very good to backup photos and music files (as you can view/listen to them from Gspace).

Add it from here : https://addons.mozilla.org/en-US/firefox/addon/1593

Here is a Video tutorial :