Looks like we don't use the storage principal currently. Instead we clear the storage for the (triggering?) principal derived from the channel: https://searchfox.org/mozilla-central/rev/a0ccd492719b1ad2106f6456549be62a76f45acb/toolkit/components/clearsitedata/ClearSiteData.cpp#161,212 Some quick tests with setting and clearing cookies via the header cookies confirmed this.
Bug 1626724 Comment 3 Edit History
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Looks like we don't use the storage principal currently. Instead we clear the storage for the (triggering?) principal derived from the channel: https://searchfox.org/mozilla-central/rev/a0ccd492719b1ad2106f6456549be62a76f45acb/toolkit/components/clearsitedata/ClearSiteData.cpp#161,212 Some quick tests with setting and clearing cookies via the header confirmed this.