After 13 years and 25 million downloads later, Firefox has officially removed FireFTP and FireSSH support from the browser. Thus, I've ended support / development of the addons. I recommend switching to Waterfox to continue using the addons.
FireFTP has a dependency on the paramikojs library so it's a couple more steps than just cloning.
git clone https://github.com/mimecuvalo/fireftp.git
git submodule init
git submodule update
or
git clone --recursive https://github.com/mimecuvalo/fireftp.git