To fix this, do the following:
Log into the Watchguard Firewall
Presuming you have already setup a Firewall proxy and are using the WebBlocker to filter content…
Go to Firewall->Proxy Actions, and EDIT the current proxy you have custom defined.
Go to HTTP RESPONSE->CONTENT TYPES
ADD a new action, (ALLOW OR AV SCAN), Pattern match, video/*
Retry playing your video (you may need to close out of your web browser and open it, and try another video, but this should solve the YouTube and other streaming video issues).