Scott Guthrie has made a post on how to change Visual Studio’s default browser:
1) Right click on a .aspx page in your
2) Select the “browse with” context
3) In the dialog you can select or add
a browser. If you want Firefox in the
list, click “add” and point to the
4) Click the “Set as Default” button
to make this the default browser when
you run any page on the site.
I however dislike the fact that this isn’t as straightforward as it should be.