Perhaps some of what works for me will help others having problems. First, I need to have Flash installed. But because I do not want flash content to load and play automatically when browsing, I have installed the FlashDisable plugin, which works wonderfully. I browse with this activated, and deactivate it with a click on the button if I need Flash on a certain webpage. With YouTube, if FlashDisable is activated when I click on the Stop Tube white screen, I get an error message (I assume it would do the same without Flash installed or if it is disabled). So I need to have FlashDisable deactivated when I go to the video I want.The white screen briefly flashes, then it changes to a window asking me to activate Flash Player. I click on it and video then plays normally using Flash rather than HTML5. Hope some of you find this helpful.
I only wish there were something like this to globally block HTML5 video from loading, rather than just with YouTube.