any internet shortcut will open which ever browser is your default browser. so here is what you need to do:
1. make sure firefox is your default browser.
2. create the internet shortcut, to do that here is the steps:
A. open the page you which to create the shortcut for.
B. copy the current URL
C. goto the desktop, right click and select new->shortcut
D. Paste in the URL and click next
E. Type the name of the shortcut, for example "my site" and click finish
your done! hope this helps