FireFTP
A secure and cross-platform FTP / SFTP client for Firefox
Pros & Cons:
Our Review:
If you use Firefox, or if you needed one more reason to consider switching, my top pick is FireFTP. After the latest round of upgrades it now supports SFTP, compression, and while somewhat hidden, even FXP (server to server transfer).
Other advanced features include directory comparison, syncing directories while navigating, SSL encryption, search/filtering, integrity checks, remote editing, drag & drop, file hashing, and more.
With this robust feature set, its ease of use, and browser integration, FireFTP should now serve the needs of all but the heaviest power user needs and is quickly becoming one of my favorite FTP clients.
If you use a browser other than Firefox or require advanced functionality not found in FireFTP then check out the "Review and Alternatives" above for great standalone applications which should meet your needs.
FireFTP was reviewed by Gizmos Freeware on

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Comments
I used to love FireFTP but sadly it is not compatible with Firefox Quantum and there doesn't seem to be an alternative. :(
Unfortunately, Firefox basically banned it. Even trying to download from the author's GitHub doesn't allow it to install. Even after changing a setting in about:config which should have allowed it through.