Localhost rendering different than live version (same browser)

I'm developing a Wordpress theme on my localhost and have started to test it on a live server. Both are viewed with FireFox. I'm a little shocked to find an image aligning different from my localhost to the live URL. I have never come across a problem like this before.

Attached is the image as it appears on my localhost and how it appears on the live server.

Code for that image that's shifting on the live version:

<div id="banner">
      <div id="banner_arrow"></div>
</div> <!-- end #banner -->

Code (markup):

#banner{background:url(img/banner_bg.png) repeat x;
width:955px;
height:73px;
position:relative;
top:90px;
left:-21px;}

#banner_arrow{background:url(img/banner_arrow.png);
width:64px;
height:120px;
float:right;
position:relative;
left:64px;
margin:-58px 0 0;}

Code (markup):

Has anyone else experienced this before??

 

I would guess it has something to do with the php. I have seen a similar problem before but could not find a good answer so I will be watching this thread too.

For someone to really help though your going to give more detail about what local server you are using, what the host is using, what versions of stuff they are using and so on.

 

Is the CSS definitely the same both local and online, you haven't changed anything and then not uploaded?

PHP version shouldn't make any difference, the browser doesn't render PHP just the HTML/CSS output