Tamarin

The Tamarin virtual machine currently implements the E C M A Script 3rd edition language standard that is the basis for JavaScript, Adobe Action Script, and Microsoft Jscript, plus some of the new language features proposed in the E C M A Script 4th edition specification... The Tamarin virtual machine will be used by Mozilla within Spider Monkey, the core JavaScript engine embedded in FireFox, and other products based on Mozilla technology.

http://www.mozilla.org/projects/tamarin/