Fetch GET Request with custom headers ReactJS

I am trying to send a GET request to a API but when i add custom headers in the code somthing strange happens. Somewhere the request method changes to OPTIONS when it reaches the web server.

But when i do the same without headers it will be a GET type. When i use the application postman (API development tool) the request works fine!

request code:

    let token = this.generateClientToken(privateKey, message);

    let myheaders = {
      "appID": appID,
      "authorizationkey": token
    }

    fetch('http://localhost:8080/api/app/postman', {
      method: "GET",
      // body: JSON.stringify(''),
      headers: myheaders
    }).then(function(response) {
      console.log(response.status);     //=> number 100–599
      console.log(response.statusText); //=> String
      console.log(response.headers);    //=> Headers
      console.log(response.url);        //=> String

      return response.text()
    }, function(error) {
      console.log(error.message); //=> String
    })

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react-dom.min.js"></script>

Server log ouput (with headers):

worker_1  | 172.18.0.4 -  17/Mar/2017:15:47:44 +0000 "OPTIONS /index.php" 403
web_1     | 172.18.0.1 - - [17/Mar/2017:15:47:44 +0000] "OPTIONS /api/app/postman HTTP/1.1" 403 5 "-" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10.12; rv:54.0) Gecko/20100101 Firefox/54.0" "-"

Server log output (without headers):

worker_1  | 172.18.0.4 -  17/Mar/2017:16:01:49 +0000 "GET /index.php" 403
web_1     | 172.18.0.1 - - [17/Mar/2017:16:01:49 +0000] "GET /api/app/postman HTTP/1.1" 403 5 "http://localhost:3000/" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10.12; rv:54.0) Gecko/20100101 Firefox/54.0" "-"

Added the NPM modules for fetch support in extra browsers:
https://github.com/github/fetch#obtaining-the-response-url
https://github.com/taylorhakes/promise-polyfill

What am i missing here? It all looks correct to me.

I am using firefox development edition to test the Reactjs app by running it with NPM start

via Steven Bakker