Scroll Anchoring in Firefox 66

Scroll anchoring works by first selecting an element of the DOM to be the anchor node and then attempting to keep that node in the same relative position on the screen. Scroll anchoring is most effective when it selects an anchor that’s small and near the top of your screen. This is one of the values of multiple browser implementations: We have gained significant experience with scroll anchoring and hope to bring that to the specification, to ensure it isn’t defined by the implementation details of only one browser.

Source: hacks.mozilla.org