Lazy loading of images through the mechanism controlled by that setting is relatively new, and many sites unfortunately still have javascript to do something similar in browsers that don't support it.
It'd be nice if Firefox could let us turn off the new feature in a way that's undetectable without specific measures to look for it, but it seems extremely unlikely that Mozilla will ever find that worth the effort of doing. In some cases you might be able to just block a single script with noscript or ublock, but all the implementations are different and there's no general solution unless maybe if you find or create an extension specific to that purpose.