A new “feature” of Mozilla FireFox 3 (Bug 433640)

When the “top center” value of the “background-position” CSS
property is used for a background image of an element that is an odd number
of pixels wide, Firefox 3 blurs the background image (No other browsers,
including Firefox 2 behave in such a manner).

https://bugzilla.mozilla.org/show_bug.cgi?id=433640