I suspect this might be connected to [bug 1737396](https://bugzilla.mozilla.org/show_bug.cgi?id=1737396) Unfortunately I haven't worked on the spell checker. Maybe Olli would know more?
Bug 1748408 Comment 8 Edit History
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I suspect this might be connected to bug 1737396 Unfortunately I haven't worked on the spell checker. Maybe Olli would know more?