I have already found one case in Firefox where this is true. FF will load the image data in background tabs, but it won't decode it (e.g. render a JPG) until you switch to that tab. I may be nitpicking but the extra half-second or so bothers me like texture pop-in in a game. So I go to about:config and set image.mem.decodeondraw and image.mem.discardable to false.