I don't have time to investigate myself (unclear if I could without a login?) but off-hand this looks like a duplicate of bug 1742629. Over to http networking component for remaining triage.
Bug 1755324 Comment 2 Edit History
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I don't have time to investigate myself (unclear if I could without a login?) but off-hand this looks like a duplicate of bug 1742629 - ie your server is probably claiming that it is sending us a `gz` file but also saying that the content is encoded as gzip, leading to the http layer decompressing it. Over to http networking component for remaining triage.