I just bought a new MacBook Pro with Retina display and I went to youtube.com in Safari to play a video, but the videos required Adobe Flash.
I downloaded Flash Player and installed it, but the videos still don't work. What can I do?
You need to remove Youtube's HTML5 trial in order for the flash player to work and remember your preferences.
First join the trial, and then leave it. Leaving the trial is the only way to force YouTube to remember you want the videos played in Flash, but of course to do that you have to join the trial first. It fixed the issue for me.
Here's the link:
If all else fails, try using other browsers (Google Chrome, Mozilla Firefox etc.)