If you're an end user, you should know that a release candidate build is better than a beta (meaning, it's been tested more and it's likely more stable), but it is still not the final version, which will probably come soon. Therefore, installation is recommended only to those curious to what the next major version of Firefox has to offer. Also, have in mind that installing Firefox 3.5 RC1 will overwrite your previous version of Firefox; your bookmarks and history will not be lost, but some add-ons might not (and probably will not) work.
Here's the rundown of the most important features in Firefox 3.5 RC1:
- 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 and elements, downloadable fonts and other new CSS properties, JavaScript query selectors, HTML5 offline data storage for applications, and SVG transforms.
- it is available in more than 70 languages