How can I force YouTube to not use HTML5 player because it is not working on the latest version of Firefox and now I can't see any video?
Solution 1:
Check if the latest version of Flash is properly installed on your browser. If not, download the latest version and install it.
Then, install the YouTube Flash Player extension:
This add-on forces YouTube to play videos using Flash Player instead of the HTML5 player.
Solution 2:
You can force YouTube to play a video with flash player by adding &nohtml5=1 or &nohtml5=True to the URL.
Found this after I had uploaded a video to YouTube and there was "&nohtml5=False" automatically added by any reason.