IMPORTANT UPDATE AS OF 2019-11-26: The jpm package and its libraries are no longer maintained as Firefox no longer supports add-ons built with jpm. If you're building a new add-on, consider a WebExtension instead and check out the web-ext tool which has all the same features as jpm. Here are some resources to help you migrate a legacy jpm built add-on.

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JPM Core

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Cross platform components for building Firefox add-ons