Datebox layout in elastic content with 3 sidebars

Howdy, and thanks in advance for any help —

I’m trying to create an elastic left column with three sidebars on the right; the first sidebar (400×200) sits above two below it (each are 200×700).

I’m having a hard time getting my first post to display correctly, specifically a custom date box.

The pertinent CSS:

#content {
	background:#101010;
	border-top: 3px double #1e1e1e;
	border-bottom: 3px double #1e1e1e;
	padding: 0 10px 10px 10px;
	margin-right: 450px;
	margin-top:-236px;
}
.post {
	background:#101010;
	border-top: 3px double #1e1e1e;
	border-bottom: 3px double #1e1e1e;
	padding: 0 10px 10px 10px;
	margin-right: 450px;
	margin-top:20px;
	}
.post-date{
	width:55px;
	height: 50px;
	background: url(images/date_button.gif) no-repeat;
	float:right;
}

The weird thing is that this is rendering “correctly” (as I want it to) in IE7, not firefox (and I haven’t yet tweaked for ie6). In IE7, the title, meta, post, and post-date will “inherit” the -236px top margin, and render in line with my first sidebar — perfect, exactly as desired.

I’m using the -236 margin to make up for the space made by the first 400×200 sidebar (open to other suggestions on this…)

But in firefox, the post and title and everything BUT my post-date box will use the -236px. The first post-date box — the first one only — will be in line with the next sidebar (236px down). All the other post-date boxes will be perfectly in line.

I’ve narrowed it down to the the “float:right” making it not be able to inherit the margin-top, but don’t see any other way to get it to play nicely.

Wow, overly long post, but does anyone have any ideas?

MANY thanks —