Ok, this works in XP......
Right click on the icon and select properties.
In the Target textbox of the Properties window, you should have the target file to launch firefox, which should be the path to your firefox.exe file, and the entire path should be in quotes.
After the trailing quote, press space once, then use this:
-new-tab about
:blank (to open a blank tab)
or
-new-tab
www.google.com (to open a tab with a specific URL)
That should work fine for you.
- Justin