Rivan,
In Firefox go to Tools and look in the Main tab, there should be a box to check which says "Check to see if Firefox is the default viewer on start-up." You can check that and when you open up either browser the next time you can make your selection to make whichever you prefer as your default browser (don't think you said which YOU prefer)...
There are some apps that will only work with IE and that may be what's happening with you...
Noel