You want to have background image specifically positioned on your page. CSS works in IE, Safari, Chrome etc. But not in Firefox. An example problem was on mechanics.astri.umk.pl, which I develop. I want to have background image (with well known Lorem Ipsum) positioned just before the logotype. The logotype has almost 400px padding-top.
The solution is here:
.front #branding{
padding: 300px 0 50px 0;
background: #000;
background-image: url(images/mechanics2010-front.png);
background-repeat: no-repeat;
background-position: left 100px; //important: pixels at the end (I think)
}
Voila! Now, the site looks similar in almost every webrowser.