My naive thinking would be that Firefox already has all the information it needs to make the policy work. When I set Firefox to ask before installing updates and then go to help, I can see the pending update. At this point, the pin-policy can decide, if the update should be installed or if it should be prevented. Usually an ESR version only gets updates for the current ESR (and not for the parallel running standard release), except when it is EOL and it is supposed to upgrade to the next ESR. So the update in the bottom scenario in the attached screenshot must be prevented, if there is a pin policy for 78.
Bug 1529943 Comment 11 Edit History
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My naive thinking would be that Firefox already has all the information it needs to make the policy work. When I set Firefox to ask before installing updates and then go to help, I can see the pending update. At this point, the pin-policy can decide, if the update should be installed or if it should be prevented. So the update in the bottom scenario in the attached screenshot must be prevented, if there is a pin policy for 78.