BUG: Content area disappears in wide screen with firefox

We are putting together a new site using the Hueman theme, and are encountering an anomaly:

When viewing the site using Firefox (v 30) on Windows 8 and a screen resolution of 1600×900 the content area for all posts disappears. It renders the left and right side bars, but no content. If I make the browser window narrower the content appears.

No problems (that we have detected yet) in other browsers or screen sizes. Nodes that are implemented as “pages” are fine. Only nodes implemented as “posts” are affected.

Here are three examples:

http://www.misailingclub.org/windsurfing-clinic/
http://www.misailingclub.org/whats-for-lunch-2/
http://www.misailingclub.org/new-members-read-this/

We have not added any code to the CSS customization.