For QA: You can test if GPC is enabled on https://globalprivacycontrol.org/. It should be always enabled in private browsing and only enabled in normal browsing when the corresponding setting from ETP settings section is enabled.
Bug 1865357 Comment 4 Edit History
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For QA: You can test if GPC is enabled on https://globalprivacycontrol.org/. It should be always enabled in private browsing and only enabled in normal browsing when the corresponding setting from ETP settings section ("tell websites not to share & sell data") is enabled.