If you start a WebAuthn register (or sign) in a debug build, then close the window while stuff is flashing, you get:

WARNING: YOU ARE LEAKING THE WORLD (at least one JSRuntime and everything alive inside it, that is) AT JS_ShutDown TIME.  FIX THIS!
Hit MOZ_CRASH(NSS_Shutdown failed) at /Users/jcjones/hg/mozilla-central/xpcom/build/XPCOMInit.cpp:769
--DOMWINDOW == 2 (0x110e2b000) [pid = 22088] [serial = 2] [outer = 0x0] [url = about:blank]
#01: XRE_TermEmbedding()[/Users/jcjones/hg/mozilla-central/obj-x86_64-apple-darwin18.2.0/dist/NightlyDebug.app/Contents/MacOS/XUL +0x54cf068]
#02: mozilla::ipc::ScopedXREEmbed::Stop()[/Users/jcjones/hg/mozilla-central/obj-x86_64-apple-darwin18.2.0/dist/NightlyDebug.app/Contents/MacOS/XUL +0x84fb63]
#03: XRE_InitChildProcess(int, char**, XREChildData const*)[/Users/jcjones/hg/mozilla-central/obj-x86_64-apple-darwin18.2.0/dist/NightlyDebug.app/Contents/MacOS/XUL +0x54cfd77]
#04: main[/Users/jcjones/hg/mozilla-central/obj-x86_64-apple-darwin18.2.0/dist/NightlyDebug.app/Contents/MacOS/plugin-container.app/Contents/MacOS/plugin-container +0xf07]

This is also probably a result of bug 1448408, where again the old visibility aborts saved us from having to think through all these states.

JC, are you planning to fix this for 68?

This won't get fixed by CryptoEng for 68; the operation that leaks is rare to run, so the priority is lower than average. We'll get it in 69.

I think this has been fixed. At least I can't reproduce it. Feel free to re-open if it recurs.