A full-page-body iframe was being sized correctly in IE but incorrectly in Firefox – it seemed to be just the default arbitrary small iframe size. Turns out the iframe had its width and height set by CSS expressions, so that it could fill up the full size of the document, minus sidebars, headers, etc. But CSS expressions aren’t supported in Firefox 2.
The solution for the width was simple, though, because the page design is fixed-width. There’s no reason to include a CSS expression when you can just set the width to an exact numeric value. It’s not like this code is reused in other locations that might have different sizes.