In firefox 48 I recently got a leaked browser.xul when I accidentally closed a window. (So I don't know exactly what I had open...) The retaining path is: $ /c/devel/heapgraph/find_roots.py cc-edges.2200.1471467236.log 0000022799F35000 Parsing cc-edges.2200.1471467236.log. Done loading graph. 000002279B807060 [JS Object (Window)] --[UnwrapDOMObject(obj)]--> 0000022799F35000 [nsGlobalWindow # 98 inner chrome://browser/content /browser.xul] Root 000002279B807060 is a marked GC object. bkelly@kosh /c/devel/tmp/cclogs/browser $ /c/devel/heapgraph/find_roots.py gc-edges.2200.1471467236.log -bro 000002279B807060 Parsing gc-edges.2200.1471467236.log. Done loading graph. via mCallback : 00000227BFE43780 [Proxy <no private>] --[private]--> 000002279CADE100 [Object <no private>] --[_actorByBrowser]--> 00000227CAA84580 [Map 2278e3ff7c0] --[key]--> 00000227D0AC0240 [Proxy <no private>] --[private]--> 000002278AFC4B50 [XULElement <no private>] --[shape]--> 00000227BFC87858 [shape] --[base]--> 000002279C504510 [base_shape] --[global]--> 000002279B807060 [Window <no private>] This is a Map() which holds browser objects alive strongly in its key: https://dxr.mozilla.org/mozilla-central/source/devtools/server/actors/webbrowser.js#257

There are a few spots that use the map's size, but I think it could be become a WeakMap relatively easily. If anyone can construct a test case that reproduces this issue, that would be wonderful to have!

I was unable to reproduce with simple steps like closing the window with devtools open. Not sure what I did! Maybe I fat-fingered a ctrl-t and ctrl-w at the same time?

Presumably [0] and [1] are supposed to clean this up in the face of tabs/windows closing. Strange that there's an executeSoon() in there... The comment about deadlocks (in C++ and triggered from JS!?) is not exactly inspiring. [0] http://searchfox.org/mozilla-central/source/devtools/server/actors/webbrowser.js#61 [1] http://searchfox.org/mozilla-central/source/devtools/server/actors/webbrowser.js#735

Moving this inactive P2 to the backlog (P3)