Yep, that often works when a version of an Addon doesn't work after a Firefox Update.
As said in the link Bald Brick posted: Editing an add-on to change its compatibility .
Simply download the Addon (.xpi), UnZip it, you'll find a number of files.
Find and Open with a text editor the "install.rdf" file.
-change the line:<em:maxVersion>4.0</em:maxVersion>
Haven't tried as yet as I don't have a great need for viewing Silverlight Videos, but this is an proven method used while waiting for an Addon developer to "Catch up".
Works a lot of the time but ...
Not sure this is the case however with Moonlight, it being such a complex 'Addon'.
Sorry haven't tested and got a positive Yes, No answer -
Worth a go anyway, thanks for reminding us of this Bald Brick