Comment 76 for bug 239826

(From update of attachment 416840)
>+ let reportURL = formatter.formatURLPref("browser.safebrowsing.malware.reportURL", true);
>+ reportURL += errorDoc.location.href.slice(12);
>+ openURL(reportURL);

Two things... First, it looks like you need to use nsIURLFormatter befor using the pref value. Second, you're in toolkit, so reading a browser pref is probably not quite right.