Borrows code from WebKit and getting prettier and faster graphics in Windows.
There has been considerable focus on JavaScript performance of the browsers in recent years, and the competition between at least three of the five most common browsers are intense. 
Microsoft has until very recently promised to make the new with the JavaScript performance of Internet Explorer, which allows the browser is behind all the others. Mozilla had Very large with JavaScript engine TraceMonkey, which came as an extension to the Spider Monkey in Firefox 3.5, but the problem is that it really only perform well under certain conditions, but performance is otherwise on the foundation itself calls from 2007 to 2008-level.
Mozilla has promised to improve the performance of Firefox up to version 4 Now, the first signals about how to do this, started to come to light.
The TraceMonkey do, is to observe the loops in JavaScript code and to convert them to the assembler that can run very fast. This is what Mozilla for "tracing". But for the other parts of the code, the JavaScript engine in Firefox much less optimized, compared with several of the competing browsers.
Now including Mozilla the advantage that both Firefox and WebKit engine, which competitors Safari and Chrome is based, is based on open source and provided with licenses that are compatible with each other. Thus, Mozilla has the ability to copy components from WebKit, if these fall in the taste.
It has now done in the Mozilla project, called the Jäger Monkey (or Monkey also Jaeger), which was started a couple months ago. The project will create a new JIT compiler (Just-in-Time) to the Spider Monkey, and this makes use of assembler from WebKit's JavaScript engine, SquirrelFish Extreme, which Apple calls the Nitro.
According to Mozilla's David Mandelin, Jäger Monkey delivers 18 percent better performance than interpreters, but that's before a series of planned optimizations have been in place. An overview of these can be found at the bottom of this document.
Mozilla's David Anderson writes in this blog post about a performance improvement in SunSpider test up to 45 percent of today's edition of Jäger Monkey.
Another Firefox-related news is that the most recent Firefox built for Windows Vista and Windows 7 now has in place the hardware accelerated rendering graphics and fonts, similar to Microsoft to make Internet Explorer 9 This is based on the programming interfaces Direct Write and Direct2D in Windows. This will provide faster and more beautiful rendition of both new and existing web content.
This support is not turned on by default in the latest Firefox buildings, but Mozilla's Asa Dotzler explains in this blog post how it all can be activated.
