FlashVideoReplacer is included by default, but it might not be an updated
version. YouTube and other video sites constantly change how they embed video
specifically to throw off things like download scripts and FVR. You can get
the latest version from http://www.webgapps.org/add-ons/flashvideoreplacer to
see if that will work.
You can also give Linterna Magica a try at http://linterna-magica.nongnu.org/
Also make sure that only one video player plugin (i.e. Totem, VLC, etc) is
active. Apparently if more than one are enabled, they will conflict.
I've noticed that Gnash does work for me, but you have to go directly to the
video page to use it. If you try to play a video straight from a user's
profile page, Gnash fails to play. I think FVR and LM do too.
