Wow, this is a great question. I can't think of any built-in setting in Firefox for this, but I see you can make some adjustments in your Netflix account settings. Could you review the following article and see whether that helps:

How can I control how much data Netflix uses?

I ran up a $350 charge on my son's wifi using Netflix now with low streaming setting it will be only 10% of that. Thank You.

I see in the System Details list that you have received the TAARexperiment extension. This extension is part of a Mozilla Shield study.

You can open the about:studies page to see currently installed Shield studies and possibly disable specific studies to opt-out.

• https://support.mozilla.org/en-US/kb/shield