I have no real clue. The assert seems to be introduced by bug 1416384 3 years ago, by Nika, but it is unlikely the regressing bug. In general it has to do with local storage, it seems, thus Jan might know something, as well?
Bug 1670644 Comment 21 Edit History
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I have no real clue. The assert seems to be introduced by bug 1416384 3 years ago, by Nika, but it is unlikely the regressing bug. In general it has to do with local or session storage, it seems, thus Jan might know something, as well?