IIRC the ThirdPartyUtil service is not thread-safe right now, (I remember stumbling across this when poking at making `nsIPrincipal` threadsafe a while back), though it appears there was an attempt to make it threadsafe in bug 1517089. We should probably fix this and make it properly threadsafe, as it looks like we're trying to use it from across threads right now.
Bug 1789503 Comment 6 Edit History
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IIRC the ThirdPartyUtil service is not thread-safe right now, (I remember stumbling across this when poking at making `nsIPrincipal` threadsafe a while back), though it appears there was an attempt to make it threadsafe in bug 1517089. We should probably fix this and make it properly threadsafe, as it looks like we're trying to use it from across threads right now. (Hmm I didn't read your comment closely enough - the XPCOM service getters probably weren't designed to be threadsafe - might be worth fixing that)