With the fix for bug 189289, junk that is auto-identified and moved to a junk folder does not trigger a biff notification. However, if a message arrives and is filtered to a folder somewhere, it is not immediately processed as junk (bug 200788). Therefore, the notification is not suppressed. See this in Moz 1.6 Final and 1.7a.
Bugzilla misfire *** This bug has been marked as a duplicate of 233930 *** *** This bug has been marked as a duplicate of 233930 ***