Rated 4 out of 5 stars
This is a great idea and should be, imo, part of Firefox built-in.
Since this effectively takes over URN handling (and other extensions could not really compete with it), I request having a customizable and expandable chart list of urns and redirects (and always make the same "%urn%", etc. wildcards available to each of these).
For example, if one wanted to add urn:foo and redirect it to example.com/%urn%, while adding urn:bar and redirecting to example2.com/%urn%, one could do it.