flashbuilder4

Why would Flashbuilder 4 work in Internet Explorer but not in Firefox?

When I produce an application in Flashbuilder 4 and click the green run button, it opens Firefox with this URI: file:///C:/chapter_1/starter/Getting_Started/PersonalTrainerWebApp_Finished/bin-debug/Main.html but the screen is blank (grey, right-click says "about adobe flash player 10") but if I copy that URI into Explorer, it is conv...