From Seo Wiki - Search Engine Optimization and Programming Languages
SpiderMonkey is written in C and contains a compiler, interpreter, decompiler, garbage collector, and standard classes. It does not itself provide host environments such as Document Object Model (DOM).
SpiderMonkey and its sister engine Rhino have implemented support for the ECMAScript for XML (E4X) standard.
Mozilla has added optimization using "Trace Trees" to SpiderMonkey. Released June 30th, 2009, Firefox 3.5 includes this new optimization technique which offers "performance improvements ranging between 20 and 40 times faster in some cases".