This video (stream, ogg container, theora, vorbis, served by icecast 2) does play in "Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.1b2pre) Gecko/20081019 Minefield/3.1b2pre":
no Fallback content
From time to time you'll see a more or less garbled preview.
Also Firefox keeps pulling the stream if you leave the page by a link or if you put an new URL in the address bar. Looks like bug 451457 is not completely dead yet.
To see the impact leave page this way and observe that the stream is still flowing even though the page is not longer open.
Side effect: The same stream URL won't play in an <video> element on the next page. Also there will be no preview. To see this effect leave page this way. Reloading the page has the same effect.