To turn on debugging output, export the following environment variable:
NSPR_LOG_MODULES="UrlClassifierDbService:5,nsChannelClassifier:5,UrlClassifierProtocolParser:5,UrlClassifierStreamUpdater:5,UrlClassifierPrefixSet:5"
and also see the <tt>browser.safebrowsing.debug</tt> pref to see debugging output from the JS pieces of Safe Browsing.