User Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.81 Safari/537.36 Steps to reproduce: Repro: http://output.jsbin.com/bemazek/ When the sticky constraint rect is smaller than the sticky element, Firefox disregards the bottom constraint entirely. This is unnecessarily harsh - the current scroll position can be such that the bottom constraint is valid whilst the top has not yet become active (see the repro for such an example). The top constraint should only become dominant once it activates. A few notes: 1. Identical behavior happens in Chrome, see http://crbug.com/715172 2. For a more correct wordage, replace every time I write 'top' with 'block-direction' and every time I write 'bottom' with 'inverse block-direction' :) 3. This probably occurs for 'left' and 'right' ('inline-direction' and 'inverse inline-direction') as well, though I haven't tested it.