Note: This applies more to site developers who have been using the new WebGL textures feature introduced in Gecko 2.0 (Firefox 4, Thunderbird 3.3 and SeaMonkey 2.1).
In Firefox 5, it is no longer possible to use cross-domain elements as the source for WebGL textures. We made this change in response to security concerns around the possibility of cross domain information leakage. Unfortunately, that means that some WebGL-using pages are no longer working. We are working with the WebGL WG on a solution to allow such pages to resume working as soon as possible — read on for details.