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.

To all the people who keep mentioning Opera: Please cease. If you like Opera, please use it without comparing it to Firefox. Opera is the pretty sports car that works only on the highways. Firefox is the burro that can take you anywhere you want to go. The two products are different. You are comparing apples to oranges, despite what you may think. Opera, despite being years older than Firefox, still cannot render thousands of sites worth a damn. It even borks Yahoo! Mail badly, something even Konqueror doesn't do. Opera still cannot work with the vast majority of online bank websites, making it effectively useless to me. Get a grip, people. If you like Firefox, quit moaning about it. The same people who moan are the same people who are not volunteering to write code or documentation, or test bugs out for Bugzilla. If you complain, be prepared to act on those complaints in a positive way. If you want a better Firefox, help make it better.