Install or Upgrade to Firefox 9 in Ubuntu

Firefox 9 was released on December 20, 2011; version 9.0.1 was released a day later and is currently the latest stable version in the "Release" channel.

The latest version of Firefox has the following changes:
 * Added Type Inference, significantly improving JavaScript performance
 * Improved theme integration for Mac OS X Lion
 * Added two finger swipe navigation for Mac OS X Lion
 * Added support for querying Do Not Track status via JavaScript
 * Added support for font-stretch
 * Improved support for text-overflow
 * Improved standards support for HTML5, MathML, and CSS
 * Fixed several stability issues
 * Fixed several security issues

Firefox 9 installation:
Open the terminal and type following command to install Firefox under Ubuntu 11.10

sudo add-apt-repository ppa:ubuntu-mozilla-security/ppa
sudo apt-get update
sudo apt-get install firefox

Once installed , you can launch the application from Unity Dash or from Applications -> Internet menu