What I'd done turned out to be very stupid indeed but I wasn't seeing an exception being thrown and because I can't find a way to host my gadget running on http://localhost in Firefox, it makes the whole debugging experience, well, how should I put this.... Let's just say "Less than rewarding".
Having removed some exception handling, hey presto, I was now getting various errors related to my parsing of the RSS feed for the Nuggets. I'd not even thought about this before because the gadget displayed the Nugget Topics fine in Firefox (also from RSS). However, the "Nuggets" RSS feed is slightly different from the "Topics" feed as it contains some extra information about each Nugget in a separate namespace. I couldn't get Start.Parser.ParseRssResponse() to pick this up so added some extra XML parsing based on selectNodes(), selectSingleNode(). These, of course are implemented in MSXML which, of course, isn't available in Firefox. Therein lies the problem.
So a few hours trying to figure out how to achieve the same thing with the Mozilla APIs and hey presto. I wont pretend it was simple. I wont pretend I didn't tear my hair out and set it on fire. I wont pretend the cat still likes me. But I did get it done.
Add this link http://www.mikeo.co.uk/gadgets/msdnnuggetfinder_mikeo.xml to your Live.com page to get easy access to the MSDN Nuggets in IE or Firefox.