I don't have Firefox 3 but look at the options, Main tab.
You should see "Always check to see if Firefox is the default browser on startup".
Uncheck it.
Then go to IE tools or Internet Properties, Program tab.
Click the "Make Default" and you can uncheck the box there to check too that way you should have IE as your default but be able to open IE or Firefox with getting bugged about it asking you anything about making the browser the default browser from either one.
You should see "Always check to see if Firefox is the default browser on startup".
Uncheck it.
Then go to IE tools or Internet Properties, Program tab.
Click the "Make Default" and you can uncheck the box there to check too that way you should have IE as your default but be able to open IE or Firefox with getting bugged about it asking you anything about making the browser the default browser from either one.