- SpiderMonkey is the code name for Mozilla's JavaScript engine. That's nothing new, as it was the first Javascript engine and was used in the Netscape browser.
Tamarin is the ActionScript engine that Adobe donated to Mozilla back in November. Tamarin has a just in time compiler that compiles Javascript down to machine code and boasts a better garbage collector. It also supports ECMAScript 3 and is working toward full ECMAScript 4 (JS2) specification support.
ActionMonkey is the code name for the project underway to integrate the SpiderMonkey and Tamarin engines. The product of this merge will be the engine for the Mozilla 2 platform. This would bring the performance improvements AND the latest specification support of Tamarin to Firefox and other Mozilla based apps.
ScreamingMonkey is an effort to get the Tamarin Engine running in non-Mozilla browsers. This brings the same Tamarin benefits to the other browsers (starting with Internet Explorer). So those other browsers are brought, "kicking and screaming" into a world with a homogenous, fast, standards compliant scripting engine implementation.
IronMonkey wants to map Microsoft's Common Intermediate Language (CIL) to ActionScript Byte Code (ABC). This would allow code written in IronPython and IronRuby to be run on Tamarin. Which, when paired with ActionMonkey and ScreamingMonkey, means running Ruby/Python code in the browser.
...in a nutshell: The non-technical reasons for choosing Tamarin are over intellectual property and licensing issues and the technical issues are related to compilation speed, file size, and memory footprint.
