Are you saying what to do or asking why it's following the scroll down reading?

I have no idea how to access the following directions Go to about:config and set media.autoplay.enabled.user-gestures-needed to false to disable all kinds of autoplay.

In Firefox the Location (address) bar where you type in www.example.com you instead type in about:config with a : colon. Then on the about:config page you can use the search bar on top to filter for the preference you want.

Though according to https://support.mozilla.org/en-US/questions/1293349#answer-1329245 The preference media.autoplay.enabled.user-gestures-needed =true has been replaced by media.autoplay.blocking_policy =1

https://support.mozilla.org/en-US/kb/about-config-editor-firefox

Autoplay settings either completely prevent pages from starting a video when the page first loads (block audio and video), or keep it muted as it plays (block audio only). Those settings cannot prevent pages from moving things around as you scroll. You might need to use an add-on for that?