blog not centered in firefox

Add this to your #container div after the background image line:

position:relative;
margin:0 auto;
width:740px;

Should look like this complete:

#container {
background:white url(images/spring_flavour/container_bg.gif) 500px 0px repeat-y;
position:relative;
margin:0 auto;
width:740px;
}

I used a ff plugin to measure the width of your blog so that may be off a little, you will need to adjust that to whatever your width is.