Can I change the headers of the HTTP request sent by the browser?

I would partially disagree with Milan's suggestion of embedding the requested representation in the URI.

If anyhow possible, URIs should only be used for addressing resources and not for tunneling HTTP methods/verbs. Eventually, specific business action (edit, lock, etc.) could be embedded in the URI if create (POST) or update (PUT) alone do not serve the purpose:

POST http://shonzilla.com/orders/08/165;edit

In the case of requesting a particular representation in URI you would need to disrupt your URI design eventually making it uglier, mixing two distinct REST concepts in the same place (i.e. URI) and making it harder to generically process requests on the server-side. What Milan is suggesting and many are doing the same, incl. Flickr, is exactly this.

Instead, a more RESTful approach would be using a separate place to encode preferred representation by using Accept HTTP header which is used for content negotiation where client tells to the server which content types it can handle/process and server tries to fulfill client's request. This approach is a part of HTTP 1.1 standard, software compliant and supported by web browsers as well.

Compare this:

GET /orders/08/165.xml HTTP/1.1
or
GET /orders/08/165&format=xml HTTP/1.1

to this:

GET /orders/08/165 HTTP/1.1
Accept: application/xml

From a web browser you can request any content type by using setRequestHeader method of XMLHttpRequest object. For example:

function getOrder(year, yearlyOrderId, contentType) {
 var client = new XMLHttpRequest();
 client.open("GET", "/order/" + year + "/" + yearlyOrderId);
 client.setRequestHeader("Accept", contentType);
 client.send(orderDetails);
}

To sum it up: the address, i.e. the URI of a resource should be independent of its representation and XMLHttpRequest.setRequestHeader method allows you to request any representation using the Accept HTTP header.

Cheers!
Shonzilla

I was looking to do exactly the same thing (RESTful web service), and I stumbled upon this firefox addon, which lets you modify the accept headers (actually, any request headers) for requests. It works perfectly.

https://addons.mozilla.org/en-US/firefox/addon/967/

I don't think it's possible to do it in the way you are trying to do it.

Indication of the accepted data format is usually done through adding the extension to the resource name. So, if you have resource like

/resources/resource

and GET /resources/resource returns its HTML representation, to indicate that you want its XML representation instead, you can use following pattern:

/resources/resource.xml

You have to do the accepted content type determination magic on the server side, then.

Or use Javascript as James suggests.

ModHeader extension for Google Chrome, is also a good option. You can just set the Headers you want and just enter the URL in the browser, it will automatically take the headers from the extension when you hit the url. Only thing is, it will send headers for each and every URL you will hit so you have to disable or delete it after use.