Update September 9th, 2014:
fakehal-0.5.14-8 below should now automatically work on Fedora 20 or RHEL7 to allow Firefox to play Amazon Instant Video.
Step 1: Install FakeHAL
Adobe Flash Player for some reason uses HAL for its DRM to function, but HAL was removed from the operating system in Fedora 16. The fakehal packages below contain a stripped down subset of hal necessary to allow this to work on newer versions of Fedora. If your firefox and flash-plugin are x86_64, then you will want the corresponding x86_64 fakehal and fakehal-libs packages.