Why do I have trouble saving videos latley?

I use Firefox with "Down Them All" and "Download Helper" plugins. Between the two I have never had any problems saving You Tube, Financial or news site videos etc, but now it seems I can save them but can't open them. I use VLC player like I always have so nothing has changed there and all software and plugins are up to date. Most files seem to always in .FLV file types which the always were.
I don't know if something has changed in video saving protocols that I have missed or what. I still get some to work but more often than not they don't.
Any suggestions would be appreciated.
Floyd