And yes it was.

Turns out I was adding a style in my code-behind, like this:

               spanImage.Style.Add("background", "url(" + imagesFolder + imgFileName + ")" + " no-repeat;");

The HTML in front, the span already had the background property set.

In IE, the browser happily interprets this as a last-one-in-wins, as so:

<span id="Template_ctl25_contentImageRight_spanImage" class="frame-video float-left" style="background-image:url(_includes/images/content/pictures/pic1.jpg);background:url(/RFL2010/images/about.jpg) no-repeat;;">

whereas FF 'chokes' (correctly?) and disregards the invalid second attempt to set the background.

There will be a genuine technical explanation for this, if anyone knows then feel free to leave a comment!