Good news: there's probably nothing you need to download! Firefox has a built-in HTML5 player and can decode many (but not all) kinds of media served to that player. (It's mostly up to websites to choose which video player they specify in the page -- Flash, Silverlight, HTML5, other -- it's not always in your control to choose what they send you.) Could you visit the following test page and see whether you get the "usual" result, which is 5 blue boxes and one red one: https://www.youtube.com/html5 It might take a moment for the test to complete. If there is a problem with the H.264 boxes (red x) then you may need to add some media features to Windows, or check whether you or one of your add-ons has modified some Firefox settings. More on that if relevant.