jpm was a Firefox Add-on SDK command-line tool that could be used to initialize, run, test, and package add-ons.
From the official website:
The Firefox Add-on SDK command-line tool that you use to initialize, run, test, and package add-ons.
jpm is based on Node.js. You can use jpm for Firefox 38 to 57. The jpm tool was the replacement for cfx. It enables you to test, run, and package add-ons.
jpm is deprecated in Firefox 53 and EOL'd in Firefox, as it was part of the Add-on SDK (which has been replaced by WebExtensions).