• Support for a new type of theme called Personas, which allow users to change Firefox’s appearance with a single click.
  • Protection from out-of-date plugins to keep users safer as they browse.
  • Open, native video can now be displayed full screen and supports poster frames.
  • Improved JavaScript performance, overall browser responsiveness, and startup time.
  • The ability for web developers to indicate that scripts should run asynchronously to speed up page load times.
  • Continued support for downloadable web fonts using the new WOFF font format.
  • Support for new CSS attributes such as gradients, background sizing, and pointer events.
  • Support for new DOM and HTML5 specifications including the Drag & Drop API and the File API, which allow for more interactive web pages.
  • Changes to how third-party software can integrate with Firefox in order to prevent crashes.

Link:
http://www.mozilla.com/en-US/firefox/3.6/releasenotes/