I've also noticed some crashes when migrating kvstore to use migrators earlier this summer, which seem to be the same issue as bug 1531887. I don't know if that's the cause of this particular crash as well, but xulstore wasn't using Migrator before either. Another thing that might fix this is bug 1681492, which vendors in https://github.com/mozilla/rkv/pull/211
Bug 1681426 Comment 3 Edit History
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I've also noticed some crashes when migrating kvstore to use Managers earlier this summer, which seem to be the same issue as bug 1531887. I don't know if that's the cause of this particular crash as well, but xulstore wasn't using Manager before either. Another thing that might fix this is bug 1681492, which vendors in https://github.com/mozilla/rkv/pull/211