CSS float bug with side pane resizing main content

We're having a problem with a side pane that is floated to the right. This pane somehow resizes the first div in the main content, so the div is stretched to the same height as the pane, see illustration below.

The main div has the following css

margin: 10px 280px 0 10px;
padding-right: 50px;

The side pane has the following css

float: right;
width开发者_运维百科: 270px;
margin: 10px 10px 0 0;

What would make this happen? How can I fix it?

This error occurs in Firefox 3.6

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Try adding overflow:auto to main div.