From bug 752994 comment 3 One problem here is a fixed position background with an offset location; this seems to move around as the user pans, which isn't intended. Testcase is at http://conduit.bitops.com/~vladimir/misc/fixed-pos-bg.html . When you zoom in/out, note that the "circle" gradient in the image starts at the same place. Zooming in/out gets it confused, and sometimes the background color bleeds through. I've made it red in the testcase so that it shows clearly. Some of the weird rendering could be because as we render different tiles, the background gets offset to the origin of the tile. You can see this same effect in desktop firefox, actually; if you open that URL, zoom in a few times, then use the scrollbar to scroll left/right... the image stays in the same spot.