Mozilla Firefox 3.5.x before 3.5.11 and 3.6.x before 3.6.7, Thunderbird
3.0.x before 3.0.6 and 3.1.x before 3.1.1 permit cross-origin loading
of CSS stylesheets even when the stylesheet download has an incorrect
MIME type and the stylesheet document is malformed, which allows remote
HTTP servers to obtain sensitive information via a crafted document.