The goal of the "Tamarin" project is to implement a high-performance, open source implementation of the ECMAScript 4th edition (ES4) language specification. The Tamarin virtual machine will be used by Mozilla within SpiderMonkey, the core JavaScript engine embedded in Firefox®, and other products based on Mozilla technology. The code will continue to be used by Adobe as part of the ActionScript™ Virtual Machine within Adobe® Flash® Player.

The Tamarin virtual machine currently implements the ECMAScript 3rd edition language standard that is the basis for JavaScript, Adobe ActionScript, and Microsoft Jscript, plus some of the new language features proposed in the ECMAScript 4th edition specification. By working on an open source implementation of ES4 with the community, Adobe and Mozilla hope to accelerate the adoption of a standard language for creating engaging Web applications. We hope the Tamarin project accelerates the ability of developers to create and deliver richer, more interactive experiences that work across multiple platforms.

I was sure there had been some discussion here of ES4, but I can't find it. Guess not.