Widevine not recognized after install by Amazon Instant (including user agent override attempt)

I followed the install instructions for Widevine per the installation page. I re-loaded firefox, and was able to see Widevine under about:addons in firefox. However, when I attempted to use the plugin to watch an Amazon Instant video, I received and error message that I was missing a DRM component (widevine). I went to about:addons per the Amazon error dialog. However, the plugin is present and is set to "always" per Amazon's suggestion. Next, I tried changing the user agent to see if the error diaglog was always presented on Linux systems. However, when changing to Windows, I received the same error message. Is there a workaround for this issue?