Considering the privacy issues (see bsdetector's comment [0]), I'm not sure they'd willingly add this. They may end up not having much choice because of the "positive" aspects of the feature being too much of an advantage in chrome, but this feature doesn't seem to fit their philosophy.

[0]: https://news.ycombinator.com/item?id=19602877 > Under "privacy considerations": "so slightly more information about the user's scrolling position on the embedding page is exposed" and "a deferred cross-origin image gets an additional piece of information about the user's scrolling position".

There is an open Firefox bug about it:

https://bugzilla.mozilla.org/show_bug.cgi?id=1535749

But the last comment raises concerns about privacy implications it may have. So, I think that right now it is unclear whether Firefox will eventually implement it and in what form.