Note that __proto__ is a property of the instances, whereas prototype is a property of their constructor functions. Ref : Mozilla Developer Network
With that it may seem that we have another property, which we can use. But the reality is “__proto__” is a custom implementation and only available toGecko Javascript engines, which is used in Firefox.
A lot advice to use this only to study the “prototype” chain of javascript but not use it in any practical application. Here is a nice explanation about the two.
Now even, Mozilla is deprecating the “__proto__” and instead suggests to use “Object.getPrototypeOf”. Here John Resig explains the new property.

Hope that helps.