Rated 5 out of 5 stars

FireFTP is an extension for Firefox that is on the level of a stand-alone FTP client.
Easy to use, comes with advanced features such as: directory comparison, syncing directories, SFTP, SSL support, search, file hasing, drag and drop, and much more.
And it's all free.

This review is for a previous version of the add-on (2.0.4.1-signed).