Today while testing the Ext JS application I was writing on various browsers, something strange happened. The app works on IE and Chrome, while on Firefox it breaks.
It turns out if not configured correctly WebAPI will return data depending on the Accept HTTP header sent by browsers, and different browsers sent different HTTP Accept header.
More details in the SO answer: http://stackoverflow.com/a/31934984
Firefox prefers XML, so Web API try to serialize data in XML format to return to Firefox.
IE and Chrome doesn’t prefer one over other, so Web API chooses Json.
I solved this issue in Firefox by configuring the web api to always return JSON by adding the following line to WebApiConfig.cs
config.Formatters.JsonFormatter.SupportedMediaTypes.Add(new MediaTypeHeaderValue(“text/html”));