Only KF8-enabled Kindles and apps support fixed layout books and, AFAIK, all of them support landscape mode. You can also force landscape mode using:
Code:
<meta name="orientation-lock" content="landscape"/>
For more information see the
Kindle Publishing Guidelines.
However, your book will of course look better in landscape mode on a 1920 x 1200 Kindle Fire than on a 800 x 600 eInk Kindle.
You may want to run some landscape mode rendering tests on eInk Kindles and tablets before deciding on a display mode.