Firefox User Extension Library (FUEL) was a JavaScript library intended for developing Firefox extensions. Co-created by Mark Finkle and John Resig, it provided JavaScript libraries and wrappers for the most commonly used operations in Firefox extensions.
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Design principles
FUEL was intended to narrow the gap between two modes of development in Firefox. One mode of development extends from the core elements of Firefox and the Firefox code base, which is predominantly based on C++. The other mode of development extends from the client-side functionality of the browser itself, which is predominantly based on JavaScript.
This latter mode of development is arguably more familiar to a wide range of users who program web applications. FUEL was intended to make extensions easier to create, especially for this latter group of people.
Status
This feature is now obsolete and replaced by Add-on SDK since Firefox 47, and only available in older release and ESR builds of versions less than 47.