User Agent: Mozilla/5.0 (Windows NT 6.3; rv:36.0) Gecko/20100101 Firefox/10000.7 Build ID: 20170617030206 Steps to reproduce: While Firefox (Nightly 56a1; Win10) was open, I shutdown my computer. Windows comes up with the "Apps preventing you from shutting down screen." Firefox is the only application listed. This has happened multiple times on both my PCs (each of which runs Windows 10 Creators Update and is on the latest release of nightly). Previously I have just ignored this and hit "Shutdown anyway". However, today I forgot to save a script I was editing, thus went back to save my changes. Actual results: I found that Nightly was "Not Responding" and, according to Task Manager was using no CPU in any of the 3 processes it had open at the time, I noted no changes in RAM, disk or network usage, the browser was doing absolutely nothing. This has happened in previous Nightly builds and I have simply disregarded it. This happened under multiple tests on both my PCs. It isn't a major issue but it certainly is one worth fixing before Firefox is released to the regualr update channel. Expected results: The computer should, ideally, shutdown as per normal and the browser stay in a responsive state so as not to interrupt a user.