Are you sure it's not just hiding the images but still loading them for layout purposes?
Is this mobile Firefox or desktop Firefox? If desktop, I'd recommend opening the dev tools and looking at the network tab there to see what it loads.
It otherwise mostly do JSON API calls, and while some are not exactly optimal, I wouldn't expect 300MB just for API calls... It's about the data usage I used to have browsing Reddit and loading a fair bit of images and websites.