It appears to be a flaw in the website's design that only occurs on Firefox. I've reported the issue to the Webcompat team (see issue #52738) so that they can look into it further and help the website fix the issue, but there's not much you can do on your end to get it working properly.

I notice some weird behavior. If I scroll down to position the image as high as possible until it just doesn't disappear then I'm able to zoom a larger part of the image.