Thank you very much for your patience, the steps that were outlined in comment 12 do clarify and explain what happened. The preference you were trying to flip back is controlled by the update server (and not intended to be set manually), so each time you made a fresh profile it "worked", until it got updated, and the updated value then got applied on restart. Please follow Bob's steps above (specifically using a new, fresh profile) to get us the required information to further diagnose.
Bug 1769811 Comment 15 Edit History
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Thank you very much for your patience, the steps that were outlined in comment 12 do clarify and explain what happened. The preference you were trying to flip back is controlled by the update server (and not intended to be set manually), so each time you made a fresh profile it "worked", until it got updated, and the updated value then got applied on restart, which you edited, which got updated again, and so on... Please follow Bob's steps above (specifically using a new, fresh profile) to get us the required information to further diagnose.