* It might be placed as a normal block element on the page but out of the current viewport.
* It might have had visibility: hidden CSS statement applied to it.
* It might have had display: none CSS statement applied to it.
* It might have the alpha channel set to fully transparent and then fade in.
* It might be inside a <div> that has overflow: scroll and then have that specific div scrolled such that the image is outside the viewport of that <div>
* It might be using top and left CSS properties to animate the position of the element such that it is off the screen.
* It might have it's CSS width or height set to 0 so that it can be start out "hidden" and then have animate open.
So I wouldn't discount this as just a simple solution because I see a lot of edge cases where things could get confusing just due to the complexity of how many ways there are for a page element to be visible versus not visible.
It comes in ten parts, and there's lots of discussion about how to deal with viewports, when to load things in, etc.
The original bug (which discarded images that weren't visible) was even worse: https://bugzilla.mozilla.org/show_bug.cgi?id=689623
Implementing these fixes in FF is easy. They didn't even have to be geniuses to realize it, just keeping an eye on friends (/rivals) would have given them the idea a few years ago.
NB: Not criticizing FF devs. They do awesome work! And sometimes, such silly little optimizations can just stay hidden at the back of everyone's minds for ages until showing themselves through a veil of obviousness.
But, cancer vs these optimizations? No chance.
Practically, cancer cannot be cured even by multiples of all the effort than went into those two bugs.