I have novell moonlight which I installed a while ago so I'm not sure it's the same thing you have but it works on the silverlight websites I've tried, in your home directory is a hidden file .mozilla dig right down to the install.rdf i.e .firstname.lastname@example.org/install.rdf
the "63ixi0xc.default" will most probably be different on your system.
open the install.rdf and look for this line
and change it to the version of firefox you have, you can do this with most addons btw, I haven't tried it but you could probably replace the version number with * and it'll never need updating.
You may then need to restart firefox
Hope this helps