Browser object is another object (Like the sprite object)
You've to add it by double clicking an empty space on the layout and then choose Browser.
After that make an action using the Browser to set Fullscreen to Scale Outer
EDIT: I just saw your image, your problem isn't about Browser fullscreen...
So you can have a better fullscreen make a bigger background in the layout (That's just to hide the white bars) .. Then re-position your User Interface items to the corresponding places.. This is really tricky so you either stick with another scaling method or learn this way.