quvi URL --exec="mplayer %u"
No, you don't have to specify mplayer's path.
Extensions always put their executables in directories already specified by
the default path environment variable.
I think you can also use the video player of your choice, but I haven't tested it.
and, dude, try using the -h flag next time.
it will take you to wondrous places.
also please spell my name right.