Use latest m-c, install cc analyzer from bug 726346.
Open Help->About Nightly
close the About Nightly dialog
click 'Run cycle collector'
Once you see the log, click 'Find Documents'.
JS implemented nsIUpdateChecker seems to keep stuff alive.
This is possibly somewhat similar problem as Bug 728230.
Created attachment 598408 [details] [diff] [review]
You probably wanted to ask review from someone.
I'm not able to reproduce the leak with Mozilla/5.0 (Windows NT 6.1; WOW64; rv:13.0) Gecko/20120224 Firefox/13.0a1
Looks pretty good on try: https://tbpl.mozilla.org/?tree=Try&rev=fbddc49d4a36