Actionscript 3 :: Bug Apparent In Browser But Not In Flash CS5?

I have a custom "StagedSprite" class that extends Sprite. Its purpose is to automatically align itself to the stage's border whenever the stage gets resized.NOW, I load a PNG file "ybmp" (Bitmap) and add it as child to "loadingIcon" (StagedSprite). In Flash CS5, the PNG is shown correctly on screen (aligned too). But if I publish and run the SWF from within Firefox or IE, the PNG IS NOT SHOWN!

What is even MORE strange is what the following code produces :

//var ybmp:Bitmap = the loaded bitmap
//loadingIcon is a StagedSprite
loadingIcon.addChild(ybmp);
trace(ybmp.bitmapData.width,ybmp.bitmapData.height);
trace(ybmp.width,ybmp.height);
trace(loadingIcon.width,loadingIcon.height,loadingIcon.getChildAt(0),loadingIcon.stage);

I put the "trace" results in a TextField on screen. In CS5, the code above produces :

215 200
215 200
215 200 [object Bitmap] [object Stage]

In Firefox and IE, the code above produces :

215 200
215 200
0 0 [object Bitmap] [object Stage]

What on earth is that?!? In the browsers, the Bitmap IS added in "loadingIcon" but "loadingIcon" still has its width and height set to zero. ?I could paste the StagedSprite code here too, but it's a little big and has no scrollRect property. After all, what could be the reason for it running in the Flash authoring tool but NOT in the browsers..?