It improves performance by compiling "non-traceable" Java Script into machine language for faster execution.Firefox 4 no longer relies on the underlying OS for text layout/shaping. This allows for smart Open Type layout/shaping which is consistent across different operating systems.Since Firefox 4.0 Beta 5, hardware acceleration of content is enabled by default on Windows Vista and Windows 7 machines using Direct2D, on OS X using Quartz (basically CPU-only), and Linux using XRender.
Users can create persistent "app tabs", and customize the tab bar, as well as the bookmark and navigation bars. Jäger Monkey is a new Java Script engine designed to work alongside the Trace Monkey engine introduced with Firefox 3.5.
As a comparison, the usage share for the Internet Explorer 9 on March 22 was 0.87%, and it was released the prior week, on March 14.
A potential factor on Firefox 4’s higher usage share is that the latter supports both Windows 2000 and XP, two operating systems Internet Explorer 9 does not support.
One of the performance optimizations was moving all application data into a single file, omni.jar, using a new file format based on the Java Archive format (previous versions used multiple files in the Java Archive format).
For later versions the file was renamed Ship twelfth revision of beta, with all beta N hardblockers fixed.
Firefox 4 has marked a major change in performance in comparison to former versions 3.6 and 3.5.