It doesnt have to be for firefox, an ftp client is completely different software than the browser. Go to google.com and search for "free ftp client". I am sure you'll find one there. Good luck, and welcome to x10!
I would personally recommend CoreFTP (google it.) However, if you would prefer to use something built into Firefox, I would recommend this nifty little extension called FireFTP: https://addons.mozilla.org/en-US/firefox/addon/684