Introductory Blog Post from Mozilla
Firefox 5 is available for Windows, Mac, Linux, and Android
The latest version of Firefox includes more than 1,000 improvements and performance enhancements that make it easier to discover and use all of the innovative features in Firefox. This release adds support for more modern Web technologies that make it easier for developers to build amazing Firefox Add-ons, Web applications and websites.
Firefox offers users complete control over the look and functionality of their Web browser with a gallery of hundreds of thousands of add-ons. About 85% of Firefox 4 users have an add-on installed to customize the look, features and functionality of their Firefox.