If you're like me, and I am, then you appreciate the ease and debugging capabilities of browsers like Firefox.  However Visual Studio continues to use IE even if you've set Firefox as your default browser in your environment.  Gladly, this can be changed, here's how.

1. In the Solution Explorer, right-click over the .aspx page such as Default.aspx to bring up the context menu.  Select "Browse with..."  This will bring up a Browse With dialog (see figure). 

2. Likely, Firefox will not be in the list. Simply press the "Add..." button, browse to the program directory (c:\Program Files\Mozilla Firefox), and select the program (Firefox.exe).

3. Press the "Set as Default" button with your preferred browser selected in the list.

4. Note, you can even set the default size of the browser window.

This it!