From Tor FAQ : Why can't I view videos on YouTube and other Flash-based sites?
YouTube and similar sites require third party browser plugins such as Flash. Plugins operate independently from Firefox and can perform activity on your computer that ruins your anonymity. This includes but is not limited to: completely disregarding proxy settings, querying your local IP address, and storing their own cookies. It is possible to use a LiveCD solution such as or The Amnesic Incognito Live System that creates a secure, transparent proxy to protect you from proxy bypass, however issues with local IP address discovery and Flash cookies still remain.
