I think this is another manifestation of the issue in this post: YouTube video won't release control when I click outside the video. However, could you test whether you can switch the focus away from the Flash plugin back to the page by clicking somewhere outside the plugin? That used to work in earlier versions of Firefox and/or earlier Flash players.

thanks for your help. previous versions for me at least didn't have this issue. any way this is clearly a bug with youtube shortcuts. maybe they changed something in the way they work with the shortcuts. If you can raise a bz it would clearly help fix this.