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YouTube moved away from Flash video years ago. I know that a small subset of people actively force YouTube to serve up Flash-based content through various tricks and plug-ins, but there is no guarantee that YouTube would continue to carry that functionality forward (it doesn't really make a lot of sense for them to run dual technologies).
Furthermore, all of the major browsers have been making changes that make it more difficult to run Flash Player, and this trend will continue to become more restrictive over time.
It's possible that you need to enable Flash Player in addition to whatever else you're doing to get YouTube to serve you HTML5 video, but the more productive path forward would probably be to figure out why YouTube's HTML5 content isn't performing adequately for you, since this is the direction in which the web is headed (and we're well along that path already). You would need to talk to your browser vendor and/or YouTube about those issues.
I'd start by making sure that Flash Player is actually working in those browsers. If it's working in other places and just not YouTube, then I'd wager that your "force Flash" mechanism of choice is busted.
If it's not working anywhere, then the following would be a great place to start: