Thanks for the response. Turns out that the Firefox 13 version I installed is 32 bit and so the Java plugin must be 32 bit as well. I had updated Java 7 to the latest 64 bit release and the plugin there was only the 64 bit version which requires a non-standard version of Firefox. At first I thought I could keep Java JDK itself as 64 bit but have a separate JRE installed that is 32 bit but I couldn’t get this configured. So I wound us using the 32 bit version of JDK 7 and this has the right plugin.
Live and Learn I guess.