Firefox is not a user friendly browser for plug-ins like scorch. The scorch program has to be installed manually in the browser's Plug In folder. Have you carefully followed the installation instructions at the bottom of the Download Viewer page?
Seeing boxes and symbols but no notes?
If you are using an older version of Firefox than more than likely it's your FireFox that's the problem. Scorch has to be installed very accurately in FireFox or it will not work. Also if you have Active X turned off it will never work. You will have to update your FireFox to the latest version and security updates as well. Even slightly older FireFox browsers are having issues installing the Active X special fonts needed to see and print the music accurately. Sibelius is working on a solution but it probably won't be in place for a week or two.
FireFox has had issues with Active X plugins since they came on the market. Problem is many media players and interactive software use Active X and it's not going to change anytime soon.
You probably already have Internet Explorer on your system: it can usually be accessed by clicking the Windows Start menu on the bottom task bar, and navigating to: All Programs > Internet Explorer.