Apart from user-visible changes, there are also a number of things in Firefox 3.6 that should help developers. From the release notes:
- 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- Protection from out-of-date plugins to keep users safer as they browse.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.
- 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.
You can get Firefox 3.6 from the download page.