Currently we are lazy loading images when the base of the viewport is scrolled past the point of intersection with the top of an image placeholder.

We can cut down on images "blinking" into view by loading them optimistically instead. This means starting the load once the bottom of the viewport approaches a certain distance (aka threshold) of the top of an image (something like 3-500px, but it will take some experimentation).

This has some advantages over loading all the remaining unseen images on a browser idle event, namely that people aren't forced to download every image regardless of whether they will ever view them.

https://hg.mozilla.org/mozilla-central/rev/fe000c5de938

I have verified that images are now loaded before reaching them as you scroll down the page (compared with the build from 2019-07-03 where you could actually see the images being loaded) on the latest Nightly 70.0a1 (Build ID 20190708213342) on Windows 10, macOS 10.14, and Arch Linux 4.14.3.