WOW! I can't believe how much time I just spent trying to change the browser I use to debug with in Visual Studio. In visual studio 2003 I could have simply gone to File>Browse With. This (at first glance) no longer appeared to me. After some research I realized that I was in a "Web Application" project rather than a "Web Site" project. This appears to mean something very different to Visual Studio. So I created a quick "Web Site" project (something I never use any more for testability reasons) but still didn't find the "browse with" option. After some poking around I found that this option was moved to the project context menu - but still only in the "web site" project. So...in a "web site" project, right click on the project, "browse with". In the window that pops up, choose add, and then browse to the firefox.exe file (mine was in in program files>mozilla firefox>firefox.exe). Then hit ok. And click the "Set as Default" button. Now you can go back to using your "web application" project!