Firefox 3.5 Beta 4 is based on the Gecko 1.9.1 rendering platform, which has been under development for the past 10 months. Firefox 3.5 offers many changes over the previous version, supporting new web technologies, improving performance and ease of use, and adding new features for users:

# Improved tools for controlling your private data, including a Private Browsing Mode.
# Better performance and stability with the new TraceMonkey JavaScript engine.
# The ability to provide Location Aware Browsing using web standards for geolocation.
# Support for native JSON, and web worker threads.
# Improvements to the Gecko layout engine, including speculative parsing for faster content rendering.
# Support for new web technologies such as: HTML5