borders in HTML

borders in HTML

I am new to HTML And CSS. In my CSS file, I have the following:
html {
border-top: 4px solid #000000;
border-bottom: 4px solid #000000;
border-left: 4px solid #000000;
border-right: 4px solid #000000;
margin:0;
padding:0;
}
body {
background: #ffffff;
color: #000000;
line-height: 50px;
font-size: 20px;
font-family: serif;
margin: 0 auto;
padding: 15px 0;
}
.bot-img {
position: relative;
bottom: 0px;
}
When I open the page in Chrome, it looks ok - like so:

When I open the page in Firefox, it looks, like so:

How would I make both match?