FireFTP is a free, secure, cross-platform FTP client for Mozilla Firefox which provides easy and intuitive access to FTP servers.

Along with transferring your files quickly and efficiently, FireFTP also includes more advanced features such as: directory comparison, syncing directories while navigating, SSL encryption, search/filtering, integrity checks, remote editing, drag & drop, file hashing, and much more!

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Filezilla is a stand-alone FTP client, cross-platform as well.
The Linux version isn't that good as the Windows one, that's why I recommend the Firefox plugin.