http://build.webkit.org/results/Windows%20Release%20%28Tests%29/r55012%20%289239%29/plugins/set-status-pretty-diff.html This is a pre-existing difference between Mac and Windows that's uncovered by this new test - Mac ignores null messages in NPN_SetMessage, while Windows treats them as empty ones. Mac Firefox sets status to "Done" when message is null - and since we don't say "Done" after finishing the load, the closest equivalent is an empty status.
Created attachment 49095 [details] proposed fix
Committed <http://trac.webkit.org/changeset/55021>.