Since updating to 17.0.71 I have video working in Firefox 9.0.1 to get it to work I simply edited the Play_Flash.html file in C:\Program Files\J River\Media Center 17\Data\Library Server\Gizmo\Default
I changed height and width from a 100% value to the values below which is pixels you can use which ever values you desire and if you want full screen right click and select toggle fullscreen.
<script type="text/javascript">
jwplayer('player').setup({
'flashplayer': 'scripts/player.swf',
'file': '[File.FlashLink]',
'autostart': 'true',
'height': '768',
'width': '1024',
'duration': '[File.Duration]',
'provider': 'scripts/jrmediaprovider.swf',
'skin': 'scripts/glow.zip',
});
Maybe the devs could query for browser type/version and use an if statement to not use % when the browser is mozilla.
Another solution would be to use the screen resolution or window size value to set the player size.
Edit: I have only tested with flv and mp4 as that is how my all my files are encoded.