Most of the time when I am developing web applications I like to test things out in Firefox first. Since Firefox has a better history with being compliant to standards I generally find that if my CSS and HTML work in Firefox then it will work in IE. Getting Visual Studio to debug with Firefox or any other browser in Visual Studio 2008 is extremely simple.

In the Solution Explorer, right click on an aspx page (I chose the startup page Default.aspx) and select Browse With. You will be presented with the following dialog:

Select the browser you want to load up when you debug the application and click the Set as Default button. That's it, we are done! When you debug you should now get a FireFox window instead. If you want to revert back to IE later on then just follow the same steps but choose Internet Explorer as the default.