Please note: Firefox 3.5 RC2 is a public preview release intended for developer testing and community feedback. It includes many new features as well as improvements to performance, web compatibility, and speed. We recommend that you read the release notes and known issues before installing this release.

New features and changes in this milestone that require feedback include:

• This beta is now available in more than 70 languages – get your local version.
• Improved tools for controlling your private data, including a Private Browsing Mode.
• Support for the HTML5 <video> and <audio> elements including native support for Ogg Theora encoded video and Vorbis encoded audio.
• 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: downloadable fonts, CSS media queries, new transformations and properties, JavaScript query selectors, HTML5 local storage and offline application storage, <canvas> text, ICC profiles, and SVG transforms.

Source: Mozilla Developers News