How to set Firefox as default client for ftp:// links?

There's a free application called RCDefault App that allows you to set the default application for different protocols, file handlers, etc.

For example, for FTP, I can set whatever application I choose to handle that protocol...

Installation is extremly easy. Once you download and open the DMG, simply copy the file to either /Library/PreferencePanes or ~/Library/PreferencePanes (if you only want it accessible to your user profile)

After it's copied, open the application up from System Preferences and you can start customizing.

It hasn't been updated in a long time, but I just verified that it works on High Sierra.