You can get faster response for PowerPress issues at the blubrry.com forums, forum.blubrry.com.
I just helped someone else with this same problem. Make sure your CSS is not styling object and embed tags. If the height is set to 'auto', and the div the object/embed is within does not have a set height, the video player will disappear in Firefox. It's bad practice to set CSS styling for object/embed tags anyway, as usually the object/embed's width and height are set at the time the embed is made, which is after the theme's CSS is developed.
For example, you have an embed for a wide screen video, the width/height will be different than than of embeds for letterbox 4:3 video. If you set these widths/heights in your CSS, you then forgo the ability to resize them on a per case basis.