Margin problems in Firefox

I’m having problems trying to line up this container box in Firefox. Again, it lines up perfectly in Internet Explorer, but doesn’t line up in Firefox.

[URL=http://img528.imageshack.us/my.php?image=ieexamplebe6.gif][IMG]http://img528.imageshack.us/img528/848/ieexamplebe6.th.gif[/IMG][/URL]
[URL=http://img100.imageshack.us/my.php?image=firefoxexamplenv0.gif][IMG]http://img100.imageshack.us/img100/6416/firefoxexamplenv0.th.gif[/IMG][/URL]

And here’s my code:

[CODE]
html, body {background-color: #ebebeb;
background-image: url(“midnight_background.gif”);
background-repeat: no-repeat;
background-position: 50% 50%;
padding: 0;
margin: 0;
height: 100%;
font-family: lucida sans;
font-size: 10pt;
color: #000000;}

#MainContainer {background-color: #ffffff;
padding: 10px;
margin-top: 45px;
margin-left: 40px;
border: solid thin #c0c0c0;
width: 880px;
height: 450px;}[/CODE]

Any solutions to make this problem disappear?

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