This functionality will be added to Firefox 75, which is scheduled for release on April 7, 2020.
On February 12, a bug opened a year ago was closed 1542784 (lazyload), which said about the impossibility of the "loading" attribute of the tag , which can take the value "lazy". It allows you to "deferred" loading of images on the page - images will be loaded only at the moment when they are in the visible area of ββthe page.
This mode should reduce page loading time, the amount of memory consumed and Internet traffic.
New features are available in nightly builds of the browser.
By default, this mode is disabled, you can enable it in about:config, for this you need to enable the dom.image-lazy-loading.enabled=true parameter.
Source: linux.org.ru