In Firefox the footer on the default (Kubrick) theme is shifted slightly, making it look pretty bad. It turns out that this is caused by the following section of Kubrick CSS:
#footer {
padding: 0 0 0 1px;
margin: 0 auto;
width: 760px;
clear: both;
}
That '1px' is the cause of the problem - it doesn't occur in Internet Explorer because IE doesn't fully conform to the CSS specification, and simply ignores the pixel shift. Changing that CSS to the following solves the problem:
#footer {
padding: 0 0 0 0;
margin: 0 auto;
width: 760px;
clear: both;
}
There has already been a Support discussion on this, and a Mozilla bug report which includes screenshots which may provide more information - but that simple CSS change would solve the problem.