Firefox 4.0 problem – is it?

My websites http://www.french-waterways.com and http://www.saintlouisbarge.com have Datafeedr sidebar ‘adverts’ (they are teasers for other parts of the site) that showed up in Firefox 3.x and show up in I.E 9.0 but they’re absent in FF 4.0.
I’m mystified and would appreciate any ideas. Thanks