Firefox not playing videos

Generally I save videos to watch them but some sites insist on watching videos in a webpage. For some reason my firefox can't play them. It says its missing a plugin. When I click to download it it says it can't find one. So what plugin am I missing? It wont play any media in a window I don't think. I am using media player classic and real alternatives for quicktime and real.

All Furryfox fags have to fall back on is "OMG THOUSANDS OF SITES DONT WORK IN OPERAH LOL LOL" and then you ask "which ones?" and they never seem to be able to reply..