This is something that a web designer / developer does. It's not a Firefox feature or controlled through user settings. However, you are right, that one could probably make an extension that overrides the original setting and forces a different behaviour.
I figured. There's probably a line in the code somewhere that defines the image, and there could be a line that tells it to scroll with the content, or remain static and not scroll off screen, but it's not there and it's piss poor design.
Fun fact: the Lemmy instance I use does do it in the Inbox, just not the news feed.
For me the image stays there the whole time.
You could try using an extension like stylus to change the css on the page to make the image behave how you like, but you'd need some knowledge of how css works.
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