Could you link to a page that has this problem? I looked at a couple demo pages and watched what Firefox loaded in the Network Monitor* and didn't see any additional images loading before I triggered them: * https://webkit.org/demos/srcset/ - triggered by zooming in or out a few times * https://scottjehl.github.io/picturefill/examples/demo-01.html - triggered by making the window very narrow (you have to widen it again after the image switches to read the entries in the Network Monitor) ''* More info:'' https://developer.mozilla.org/docs/Tools/Network_Monitor