Shared components used by Firefox and other Mozilla software, including handling of Web content; Gecko, HTML, CSS, layout, DOM, scripts, images, networking, etc. Issues with web page layout probably go here, while Firefox user interface issues belong in the Firefox product. (More info)
In Youku's HTML5 player, fullscreen doesn't work as expected when unprefixed Fullscreen API is enabled.
It is currently unclear what is happening in this case, but we would need to investigate this in order to ship the unprefixed API.