STR 1. create a new profile, then disable "Block pop-up windows" in Preferences 2. restart the browser, then load http://lcamtuf.coredump.cx/cross_fuzz/cross_fuzz_randomized_20110105_seed.html NOTE: this test may open a lot of windows, so I suggest you run it in a VM, or if you're on Linux run it with a separate X server like Xephyr.
My guess is the mReadyPromise is being cycle collected just before we try to access the promise again somehow. Or somehow we are getting another ServiceWorkerContainer created somehow. The safest fix here is to probably change this assert: https://searchfox.org/mozilla-central/source/dom/workers/ServiceWorkerManager.cpp#1415 Into code that returns the existing promise if there is an entry for the window. Probably can't easily write a test for this one. Eden, do you want to look at this one as well after your current assertion fix bug?
Yes, I will work on this bug after fix the current assertion bug.