Back in the day there were web browsers. You used them to visit sites and, umm, browse the web. Then came Firefox. Sure, you can still use it to look at web pages, but with the advent of Add-ons, now you can do all kinds of extra stuff, from analyzing the structure of web pages to setting-up to-do lists, and more.
FireFTP is a full-blown FTP client that comes as a Firefox Add-on. Once you go through the install process, you've got a fully functioning FTP tool to use for file transfers to- and from your system. It supports both plain vanilla FTP transfers, as well as encrypted SFTP uploads and downloads. Even though the browser and Add-on are platform independent, you will need to go through some extra machinations to get SFTP to work on your Mac or Linux system. It's got enough options to let you customize its behavior in ways that make the most sense to how you work, including saving login info, default directories, and such.
FireFTP is a free download. Since it's a Firefox Add-on, you'll need the Firefox web browser (version 3 or later) to use it.