From listserv traffic:
"I build de demo script (for firefox) here : http://experiments.benvii.com/blob_content_length/
You will also notice that the player's load event isn't called.
Content-Length should be added to firefox (maybe open a ticket on https://bugzilla.mozilla.org/) but it should also be recommended in the section "11.7.3. Request and Response Headers" of the spec."