The latest version of Firefox has the following changes, although the final change log will be made public on 21st June, when it is officially launched:
Added support for CSS animations
The Do-Not-Track header preference has been moved to increase discoverability
WebGL content can no longer load cross-domain textures
Background tabs have setTimeout and setInterval clamped to 1000ms to improve performance
Improved canvas, JavaScript, memory, and networking performance
Improved standards support for HTML5, XHR, MathML, SMIL, and canvas
Improved spell checking for some locales
Improved desktop environment integration for Linux users.

After downloading Firefox 5, make sure all your plugins are up-to-date here. Adobe Flash has recently released a critical security update. Do ensure that you are running the latest version.

Support and hel.p for Firefox 5 is available here.

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