Video not found on the website

Hello friends, can someone faced with such problem: recently, somewhere a week ago, stopped playing videos on the site, playground, writes the error error loading plugin: plugin file not found, although I'm on your Firefox plug-ins no is not removed, and new ones are not put for quite some time, but the point is that play doesn't want and chrome, Yandex, and here Opera and eksploer reproduce without problems. With YouTube and other video sites plows without problems. Can someone tell me what settings are lost (in aplikeyshen or programs date or somewhere else), who faced????