I have been using Visual Studio 2005 recently on a machine that I didn't set up. The machine when I arrived was configured to use Firefox as the default browser - I like Firefox just the performance testing that I needed to do had to use IE.
How can you change it? I searched the project/solution settings then I searched the IDE settings and finally, the registry. i found the answer here. In essence, right click the default.htm page and choose "Browse With"; you are then presented with a menu of installed browsers and can set the default.
In case any one wants to suggest it, uninstalling Firefox does not work as the IDE only reads the default browser at install time, and at no time after that!!!!