Firefox 3.6 Release Candidate is built on Mozilla’s Gecko
1.9.2
web rendering platform, which has been under development for
several
months and contains many improvements for web developers, Add-on
developers and users. This version is also faster and more
responsive
than previous versions, and has been optimized to run on small
device
operating systems such as Windows CE and Maemo.

• Made some changes to how
  we allow third party software to integrate with Firefox in order to
  prevent crashes.
• This release candidate is available in more than 70
  languages – get your local
  version.
• More than 70
  fixes from the last beta to improve performance, stability,
  security and features.
• The ability to run scripts
  asynchronously to speed up page load times.
• Users can now change their browser’s appearance with a
  single click, with built in support for Personas.
• Firefox 3.6 will alert
  users about out of date plugins to keep them safe.
• Open, native video can now be displayed full
  screen, and supports poster frames.
• Support for the WOFF font format.
• Improved JavaScript performance, overall browser
  responsiveness and startup time.
• Support for new CSS, DOM and HTML5 web technologies.

Developers can find out about all
the changes and
new features at the Mozilla Developer Center.