When a small preview is pending, and the dispatch comes back saying it's been completed, the preview widget replaces the img tag with an identical img tag. Except in Firefox, this causes the preview to display. In firefox, the gray image remains cached, even after navigating to other pages with that dataset thumbnail on them.

There are aggressive no-cache directives on the images, and if you right-click on the preview and select "view image" FF will show the new image, rather than the gray one.