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?

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.

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.

Firefox has release MSE support for HTML5 playback. This uses the system hardware decoders unless the driver is blacklisted. Setting media.windows-media-foundation.use-dxva=false in about:config will use software decoding.