This page is for bug 907275. You can see that the scroll events fire properly if there's room for the page to scroll (this page is 2000px tall). Scroll the page in Mac Firefox 23 using your trackpad scroll gesture and note the numbers in the dev console. If your gesture results in actual page scrolling, the DOMMouseScroll events come out nice and evenly. If not (e.g. if you're at the top or bottom of the page), they don't.