I am less sure this is a fission specific issue or not. Attaching file is a patch including browser mochitest which causes this assertion. I am going to a backtrace for the assertion in the next comment.
Bug 1574100 Comment 0 Edit History
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I am less sure this is a fission specific issue or not. Attaching file is a patch including a browser mochitest which causes this assertion. I am going to post a backtrace for the assertion in the next comment.