From MozillaZine Knowledge Base

I found the check-in the added the pref, but not the first instance of it being used. It was added to /xpfe/components/bookmarks/resources/bookmarks.js later, and it existed in /xpfe/communicator/resources/content/contentAreaClick.js since before that file was created. --Np 16:44, 24 November 2005 (PST)

Looks like it was originally added to /xpfe/browser/resources/content/navigator.js -- I'll add the link. (Searching Bonsai for all checkins made by Akkana on 2000-03-13 made it easier to find.) --Unarmed 18:28, 24 November 2005 (PST)