In bug 838711 we found some leaks in Metro Firefox tests, caused by the DOMContentLoaded listener added here but never removed: http://hg.mozilla.org/projects/elm/rev/cbb24f3d4d11#l1.106 That file is forked from nsLoginManager.js (and split into parent and child scripts, for e10s support). The original code is here: http://hg.mozilla.org/mozilla-central/file/325397090d12/toolkit/components/passwordmgr/nsLoginManager.js#l252 At first glance, it looks like the toolkit code might cause a similar leak, though I'm guessing I might be wrong since no leaks are detected in desktop Firefox tests. Maybe it's different because nsLoginManager is a singleton, while the Metro LoginManagerChild.js is a frame script, loaded once per tab? Anyway, I'm opening this follow-up bug as a reminder to double-check whether desktop has a similar issue (and if not, to understand why).
See some relevant discussion in bug 839280 comment 14.
It looks like that bug fixed the leak mentioned here. I guess I should have investigated this sooner. :/