I posted a thread on the MDN discord: https://discourse.mozilla.org/t/scrolling-behavior-on-mdn/32...
Edit: Another person pointed out that it's caused by the scroll-behavior: smooth; style. I'm guessing they won't change the MDN website and the extensions will need to ignore/disable the style through CSSOM.
The property docs say "User agents are allowed to ignore this property.", so I guess that makes sense.