See if any of these links can help you. https://support.mozilla.org/en-US/search?esab=a&w=1&q=HTML5

Or if you want to stop the player,

Flash block {web link} Never be annoyed by a Flash animation again! Blocks Flash so it won't get in your way, but if you want to see it, just click on it

Nope, none of the links deals with my question.

And it's not the Flash plug-in I'm concerned with, it's the built in HTML5 audio/video playback.

I'm looking for a configuration option to switch that off.

This and other such add-ons will also stop HTML5 players.

https://addons.mozilla.org/en-US/firefox/addon/flash-control/ Controls as and when to display the Flash player and the HTML5 player.

FredMcD said

This and other such add-ons will also stop HTML5 players.

Unfortunately, this is not true.

Flash is an external plug-in. The HTML5 player ist built-in. It runs with and without Flash.

I don't have Flash running. And, still, videos run.

In about:config I have disabled:

• media.autoplay.enabled
• media.directshow.enabled
• media.encoder.webm.enabled
• media.ogg.enabled
• media.opus.enabled
• media.webm.enabled
• media.windows-media-foundation.enabled

Still, some videos run.

YouTube stopped working, though, but some advertisement videos still run.

You can also try this extension:

• FlashStopper: https://addons.mozilla.org/firefox/addon/flashstopper/