The Kindle Fire HD has Screen Rotation enabled by default. The screen will automatically flip to landscape or portrait view depending on how you hold it. If you’re reading a book or browsing the web, it can be annoying — here’s how to turn it on or off.
Swipe from the top of the screen down to bring up the Notifications screen and tap the Lock / Unlock icon.
You can do this in portrait or landscape mode. To turn rotation back on, just tap the icon again.
When your watching a movie or playing a game, it will automatically stay in its default orientation, so you won’t need to worry about locking the screen then.