Open System Preferences.
Click "Desktop/Screensaver".
Choose "Desktop" tab.
Drag the image from Firefox to the placeholder image in System Preferences.
Ta-da.
*This doesn't save the image to your hard drive in an easily accessible place, so if you'd like to keep it afterwards, follow RIDE's advice.
Select the size you want from the dropdown (if you have an iMac: 1680 x 1050) on the interfacelift site... then click download... then save it where you want it... then follow the steps above.
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