See the URL. Using that plugin, users can set a pref to clear the "New: " off of subjects from the first mail about any bug, thereby fixing GMail's threading behavior to include that first mail in the same conversation as later mails.
The GmailThreading extension doesn't exist anymore. Instead, the bmo guys are waiting for a different bug to be resolved: https://bugzilla.mozilla.org/show_bug.cgi?id=663747 I was talking about it with Christian (idl0r) today. Add me to the list that wants to see this feature.
Fair enough, thanks!
This may be useful to anyone stumbling on this request: http://blog.jolexa.net/2011/10/24/tip-intelligent-bugzilla-mail-threading-in-gmail-using-procmail/
This should be fixed through the update to Bugzilla 4.4.