If you put an iframe inside a div:
<div id="big-box">
<iframe src="http://www.google.com" class="frame">
</iframe>
</div>
And use CSS to ask the iframe to occupy all the available screen area:
#big-box {
background-color: #aaf;
}
.frame {
border: medium none;
height: 100%;
width: 100%;
}
It works perfect on Chrome (12.07) but it fails on Firefox (3.6.17). This demo shows that in Chrome the entire JSFiddle area contains the iframe, but if you try this sample in Firefox the iframe will occupy all width, but not all height.
Note also that if you fix width and height for big-box it works, but this is not the point (I can't change this). The point is why this incompatibility and how to fix it (do always as Chrome does).