JägerMonkey - also known as JaegerMonkey - is a marriage between a modern, fast Just-In-Time (JIT) compiler alongside the TraceMonkey engine which was developed for Firefox 3.5. While current versions of Firefox rely on the TraceMonkey engine - which attempts to find the most-used code fragments in a site and compiles them into native code for a massive boost in speed - for their performance, the underlying engine that is used in the event that TraceMonkey is unable to do its magic doesn't really stand up to comparison with the performance from rivals such as Chrome and Opera.
Mozilla's David Anderson didn't think that relying on TraceMonkey was enough. Quoted over on Softpedia
as asking "Why couldn’t we trace and keep going SUPER AWESOME FAST, and when tracing fails, fall back to STILL REALLY FAST,
While the project is in the first stages of development at the moment - expect to see something in Firefox 4.0 at the absolute earliest - the new JIT compiler Anderson has developed is currently performing around 30 percent faster on 32-bit hardware and a whopping 45 percent faster on 64-bit hardware than its predecessor. Further development by the Mozilla team on the native-code assembler used by TraceMonkey - Apple's Nitro compiler - is also yielding gains of around 18 percent.