Just purchased an MBP Retina today. Im fully charged at 100%. I have wifi switched on, bluetooth off, Firefox running with around 20 tabs open. I also have the App Store in the background (hidden) downloading Yosemite. My screen brightness is slightly higher than 50%. However, the battery display says I have only 5 hours and 30 minutes left. https://www.apple.com/macbook-pro/features-retina/ says I should be having something close to 8 hours... Is this normal?