[4] The engine powers the Firefox web browser and has used multiple generations of JavaScript just-in-time (JIT) compilers, including TraceMonkey, JägerMonkey, IonMonkey, and the current WarpMonkey.

It is the first JavaScript engine, written by Brendan Eich at Netscape Communications, and later released as open source and currently maintained by the Mozilla Foundation.

Its design allows it to be embedded in applications beyond web browsers, with implementations including MongoDB database system, Adobe Acrobat, and the GNOME desktop environment.

)[6] In late 1996, Eich, needing to "pay off [the] substantial technical debt" left from the first year, "stayed home for two weeks to rewrite Mocha as the codebase that became known as SpiderMonkey".

[13] Instead of compiling whole functions, TraceMonkey was a tracing JIT, which operates by recording control flow and data types during interpreter execution.

[14] JägerMonkey, internally named MethodJIT, was a whole-method JIT compiler designed to improve performance in cases where TraceMonkey could not generate stable native code.

Although this prohibits optimizations that require instruction reordering, JägerMonkey compiling has the advantage of being very fast, which is useful for JavaScript since recompiling due to changing variable types is frequent.

A more in-depth explanation of the technical details was provided by Chris Leary, one of SpiderMonkey's developers, in a blog post Archived 9 December 2012 at archive.today.

[18] IonMonkey was a more traditional compiler: it translated SpiderMonkey bytecode into a control-flow graph, using static single assignment form (SSA) for the intermediate representation.