You will also need the Addon FireFTP, If you do not already have this addon then you can get it by clicking on your tools drop down in the main menu and then selecting the Add-ons link
From the pop up window search for FireFTP and when it has loaded in the list click the 'Add to firefox' button
Once the add on has been installed you can access it from the tools drop down
Once you have FireFTP open in firefox you will need your login details... login to your website and select 'Classic FTP Login Details' from the "Manage Files" popout
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From the window that opens take note of the details shown in image below, these details will be specific to your web space - Note: www.cms-tool.net is what you use if you dont have a domain name.
Back in firefox click on the dropdown and select create account and from the popup window that appears enter your FTP details in the appropiate boxes and press ok.