In Chrome, Firefox and Firefox Mobile is a responsive layout that varies depending on width. The measurement of webpage width (window.innerWidth) in Firefox 22 and later is different than in earlier versions because it is adjusted based on the zoom level, as part of Firefox's becoming a "DPI-aware" application. 1260 physical monitor pixels will be reported as a lower number if your zoom level is higher than 100%. For example, if the zoom level is 125%, it is reported as 1050 CSS layout pixels.