Ever develop a web site that displays different content based on the visitor’s browser’s language settings? The server side code would look for the accept-language header and then do it’s magic. But how do you test this easily. Introducing Modify Headers for Firefox. Simply setup a new header with these parameters and you can start browsing as if you spoke Spanish (or whatever other language you prefer):

Action: Modify
Name: accept-language
Value: es-mx,es-us,es