Firefox 4.0

What's new in this version:

- Firefox 4 is available in over 80 languages
- Uses JägerMonkey, a new, faster JavaScript engine that is up to six times faster than Firefox 3.6
- Support for the Do Not Track ("DNT") header that allows users to opt-out of behavioural advertising
- Firefox Sync is included by default, allowing you to securely synchronize between multiple computers and mobile devices
- Certain graphics rendering operations are now hardware-accelerated using Direct3D 9 on Windows XP, Direct3D 10 on Windows Vista and 7, and OpenGL on Mac OS (OpenGL on Linux will be supported in the future)
- Direct2D Hardware Acceleration is now on by default for Windows 7 users
- WebGL is enabled on all platforms that have a capable graphics card with updated drivers
- Native support for the HD HTML5 WebM video format, hardware accelerated where available
- Firefox button has a new look for Windows Vista and Windows 7 users
- Tabs are now on top by default on Windows, Mac OS X, and Linux
- You can search for and switch to already open tabs in the Smart Location Bar
- The stop and reload buttons have been merged into a single button on Windows, Mac OS X, and Linux
- The Bookmarks Toolbar has been replaced with a Bookmarks Button by default (you can switch it back if you'd like)
- Crash protection for Windows, Linux, and Mac OS X when there is a crash in the Adobe Flash, Apple Quicktime or Microsoft Silverlight plugins
- You can turn any tab into an "App Tab" by right-clicking on it and selecting "Make into App Tab" from the context menu
- The default homepage design has been refreshed
- Overhaul of the bookmarks and history code, enabling faster bookmarking and startup performance
- Per-compartment garbage collection is now enabled, reducing work done during complex animations
- Additional polish for the Firefox Add-ons Manager
- Improved web typography using OpenType with support for ligatures, kerning and font variants
- Web developers can animate content using CSS Transitions
- Responsiveness and scrolling improvements from the new retained layers layout system
- HTML5 Forms API makes web based forms easier to implement and validate
- Support for the new proposed Audio Data API
- Support for HSTS security protocol allowing sites to insist that they only be loaded over SSL
- A new feature called Panorama gives users a visual overview of all open tabs, allowing them to be sorted and grouped
- An experimental API is included to provide more efficient Javascript animations
- Firefox now supports the HTML5 video "buffered" property
- Changes to how XPCOM components are registered in order to help startup time and process separation
- New Addons Manager and extension management API (UI will be changed before final release)
- Significant API improvements are available for JS-ctypes, a foreign function interface for extensions
- CSS Transitions are partially supported
- Core Animation rendering model for plugins on Mac OS X. Plugins which also support this rendering model can now draw faster and more efficiently
- Web developers can update the URL field without reloading the page using HTML History APIs
- More responsive page rendering using lazy frame construction
- Link history lookup is done asynchronously to provide better responsiveness during pageload
- CSS :visited selectors have been changed to block websites from being able to check a user's browsing history
- New HTML5 parser
- Support for more HTML5 form controls
- Web authors can now get touch events from Firefox users on Windows 7 machines
- A new way of representing values in JavaScript that allows Firefox to execute heavy, numeric code (used for things like graphics and animations) more efficiently