Hi Greg, It is 99.9% certain that it is what Karl said. Here is what has become my set spiel on the subject:
Mac Firefox has two text rendering anti-aliasing modes, one of which makes the text look much lighter or less bold in weight. The usual mode is used when everything on the page has an opacity of 1, or fully opaque. The moment anything drops to 0.9999 opacity, as it does on fadeOut (although this is not a jQuery issue - it's any form of opacity) all the text on the page shifts to the lighter text rendering mode. The reverse also occurs on fadeIn and also between .0001 and 0, that is, just as something becomes completely transparent. It's annoying and the only two workarounds are (1) to set opacity: .999 on the body element which forces all the text on the page to use the light rendering mode 100% of the time and never switch to the bolder mode. This sometimes looks quite good when the design has dark text on a light background. It wouldn't do for light text on dark though, as the text just becomes far too thin and begins to lose readability. The other (2) workaround is to avoid using opacity in the animation completely and using a simple slideDown instead. Joel Birch.
