I suspect this to have the [same underlying problem as here](https://bugzilla.mozilla.org/show_bug.cgi?id=1611094#c18).
Bug 1638170 Comment 6 Edit History
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~~I suspect this to have the [same underlying problem as here](https://bugzilla.mozilla.org/show_bug.cgi?id=1611094#c18).~~ No, here we already use a ThreadSafe variant.