What is the JavaScript engine used in Firefox?

The JavaScript engine used in Firefox is SpiderMonkey. It is Mozilla’s open-source JavaScript and WebAssembly engine, integrated with Firefox’s Gecko browser engine to execute scripts and support modern ECMAScript features.

SpiderMonkey originated at Netscape. Brendan Eich created the first JavaScript engine in 1995 while developing JavaScript for Netscape Navigator; the implementation later became known as SpiderMonkey. Mozilla now maintains it as a standalone engine that can also be embedded in other software, although Firefox remains its best-known host.

The main mix-up is confusing browser engines with JavaScript engines. Gecko handles much of Firefox’s page rendering and browser platform work, while SpiderMonkey executes JavaScript. Google Chrome and Chromium use V8, Safari uses JavaScriptCore, and the former Microsoft browser engine Chakra is no longer Firefox’s engine. SpiderMonkey has evolved through several just-in-time compiler generations, including TraceMonkey, JägerMonkey, IonMonkey, and WarpMonkey, improving performance while preserving the ECMAScript language model.