I haven't really tried much, but one thought is to enable YouTube's "HTML5
trial", which will use the alternative player for HTML and Javascript for
videos that don't have ads on them.
Another possibility if Midori supports custom user scripts (I don't know if
it does or not) is ViewTube, which is a Greasemonkey script that replaces the
regular YouTube video player with a different one that lets you select the
quality and format and play it with the web browser's capabilities. This is
usually how I watch YouTube videos on Abrowser.
I'll leave any information on how to get Gnash to work to someone else, since
I don't really know about that.
