This page demonstrates Firefox 3.6 and Opera 10.5 failing to display fallback content when MP4 video is not supported. This particular example uses the following HTML:
<video id="orders-video" width="480" height="360" controls>
<source src="/video/orders.m4v"></source>
<object data="/video/mediaplayer/player.swf" type="application/x-shockwave-flash" width="480" height="360">
<param value="/video/mediaplayer/player.swf" name="movie"/>
<param value="file=/video/orders.m4v" name="flashvars"/>
</object>
</video>