Like many users, my experience with Adobe's Flash Player hasn't been a happy one, and it's gotten worse as Adobe has "improved" the player over time. Video playback using the Flash Player in both Firefox and Internet Explorer has gotten so bad for me on Windows that I've switched to Google Chrome for viewing Flash video. Adobe has apparently been paying attention to the criticism, and the beta of Flash Player 10.2 November 30th that the company released on its Adobe Labs website is the first version in a long time with significantly better video performance for virtually all users, not just those with dedicated H.264 acceleration.
Flash Player 10.2 is the first public implementation of Adobe's Stage Video architecture, which makes better use of whatever video acceleration (usually in the GPU) is available in the user's computer. Adobe claims that Stage Video decreases CPU usage by up to 85%. As a practical matter, Flash videos are playing cleanly, without stuttering or dropouts, on Windows using Flash Player 10.2 in both Firefox and IE. Windows 7/IE 9 users will also benefit from graphic acceleration using any available hardware rendering capabilities. If you're using Flash Player 10.1 or earlier and you don't need to continue using it for development purposes, I strongly recommend uninstalling 10.1 and replacing it with 10.2.