4 color variations
Mozzilla wanted to test the color of the download button on the Firefox page.
Changing the color of the download button from Green to Orange will increase downloads.
This is kind of the mother of all AB tests: change the color of the call to action. Generally the most contrasting color would perform best. The original green color converted best. Note that the traffic on this page is already very much focussed on the task of downloading Firefox. Most conversion optimizers would be very happy with a 75% conversion rate.