Well, OPFS is still disabled by default on all branches, so the possible security issue could be only observed if someone enables the pref manually. I gave it P1/S2 just to get this higher priority over other OPFS bugs which are mostly P2/S3. Uplifting the fix would be nice to have, but it's not a must IMO.
Bug 1790207 Comment 6 Edit History
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Well, OPFS is still disabled by default on all branches, so the possible security issue could be only observed if someone flips the pref manually. I gave it P1/S2 just to get this higher priority over other OPFS bugs which are mostly P2/S3. Uplifting the fix would be nice to have, but it's not a must IMO.