The Firefox 4 Beta includes the Feedback Add-on which includes you in user studies. No data will be sent to Mozilla without your permission.

It's going to be available for Max & Linux users like before, myself can't wait for the actualy final release

Sneek Peek at the final theme:
http://downloadmozila.com/wp-content...screenshot.png

More Info:
Firefox 4 Beta

Download:
Firefox Nightly Builds