Let's take the following object:

{
"isbn": "123-456-222",  
"author": {
    "lastname": "Doe",
    "firstname": "Jane",
    "coAuthor": {
        "lastname": "Johnson",
        "firstname": "Jim"
    }
},
"editor": {
    "lastname": "Smith",
    "firstname": "Jane"
},
"title": "The Ultimate Database Study Guide",  
"category": ["Non-Fiction", "Technology"]
}

What's the best way to delete/remove all instances of the firstname key(s) in javascript?