- Right click on a .aspx page in your solution explorer
- Select the "browse with" context menu option
- In the dialog you can select or add a browser. If you want Firefox in the list, click "add" and point to the firefox.exe filename
- Click the "Set as Default" button to make this the default browser when you run any page on the site.
Setting a Default Browser for Visual Studio 2005/2008
Setting a Default Browser for Visual Studio 2005/2008 :