Ads obscuring greybox effect in FF3

[RJM62]

I have a site I'm working on that uses the greybox effect for images, and also has eBay ads (Flash). The problem is that when an ad is in the viewport and a thumbnail is clicked, the ad obscures the greybox. This doesn't happen in IE7 (and I haven't yet tested it in 8).

To see what I mean, go to the following link and scroll until Robert Moses is visible, and part of the eBay ad above it is also visible, and then click on Robert's mug.

http://www.shunpikers.com/avoidance.html

The image opens properly in the greybox, but the ad obscures the greybox and remains in its position relative to the page (that is, it scrolls with the page).

I have tried a few things, including setting the z-index for the div containing the ad to a low number. No joy.

The CSS for the ad box is:

div#rightbox_310 {
    float: right;
    width: 310px;
    padding-top: 5px;
    padding-bottom: 5px;
    padding-left: 10px;
    margin-left: 5px;
    border-left: #FFCC66 1px solid;
    text-align: center;
    font-size: 10px;
    z-index: 1;
}

The CSS for the greybox assigns z-indices no less than 100.

Any ideas?

Thanks,

-Rich