IE6 vs FF2 problem with Zen derivative theme

Hi,

I am working on finetuning our theme that we based on the Zen example theme (both in layout as well as 'files on disk'). It is already online and you can find it at http://www.pegasusvolleybal.net/.

Problem: when you compare IE6's and FF's rendering, you will notice that the title bar of the topmost menu block in the righthand column of blocks/menu's is missing in IE6.

Prelim analysis: I have a vague gut feeling that this is probably a boxmodel problem because when I add even the tiniest 1px bottom-padding, all menu and block titles disappear. However, I have been looking at the css for several evenings now and I really cannot find anything of setting a height (related to bottom padding) for either the entire column or for that particular block.

The site validates in terms of HTML and CSS (w3c validators) so it doesn't appear to be a bug of forgetting to close a div or something like that.

From what I have been reading about boxmodel and quirks/standards mode, from my analysis of the doctype thingy, I think IE6 should run in standards mode (and does that then imply that the problem is not related to the boxmodel issue)?

Help wanted: I am looking for an experienced designer that is willing (via the web, firebug, etc) to look at the site and analyze the problem. I am at a loss here; I dare not (yet) to reduce the site to bare bones to find in this way what could cause the problem because my knowledge of css and drupal theming is really not all that.

Thanks for your time!