Since just the pref-flip would be acceptable for 78 I don't want to give up on this yet without talking to the WebExtensions team. Better to get that in than have to ship a Normandy update later (which, due to this bug, won't reach the affected people!).
Bug 1308251 Comment 50 Edit History
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Since just the pref-flip would be acceptable for 78 I don't want to give up on this yet without talking to the WebExtensions team. Better to get that in than have to ship a Normandy update later ~(which, due to this bug, won't reach the affected people!)~. [updated: Normandy uses a completely separate mechanism and isn't affected]