To change the default browser in Visual Studio you have to right-click on any ASPX page within a Web Project (Web Site) and click “Browse With…”.
Changing Default Web Browser
So, a Browse With Window will open. Then you can add new browser or set an existing browse as default.
Browse With Window
Visual Studio Express – Target Platform
Visual Studio – Debug Release Mode
ASP.NET – Firefox pedindo senha no IIS, IE e Chrome não
Scott Hanselman – How to change the default browser in Visual Studio programmatically with PowerShell and possibly poke yourself in the eye