Created attachment 822783 [details] testcase With: user_pref("media.mediasource.enabled", true); this testcase causes a shutdown hang with 100% CPU use. Looking at the sample, I guess the "Media State" thread can never shut down because there are always more events for it?
Created attachment 822784 [details] hang sample I used Activity Monitor to get a sample of Firefox Nightly during the hang.
The testcase in bug 975782 is a superset of this testcase.
No longer hangs. Unsure which bug fixed this as there have been many changes since the bug was filed. Added the testcase to crashtests to catch future occurrences.