The bouncing unicorn easter egg

Since Firefox 29+

• click the "hamburger" icon to enter the menu panel
• enter customize mode ("Customize")
• drag away from the menu panel all the icons, ("Cut Copy Paste" and "- 100% +" included), exit customize menu
• open again the menu panel and admire the mighty bouncing unicorn. If you mouse hover on it, you'll see (s)he has amazing rainbow mane and tail