The purpose of bug 1657588 is to clarify what happens when a user clicks "cancel" on that dialog - namely, that Firefox will not send a client certificate. If the "remember this decision" box is checked, Firefox will remember that decision to not send a client certificate. If a user wants to user a different certificate, they'll have to delete the saved decision in the certificate manager.
Bug 1729700 Comment 4 Edit History
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The purpose of bug 1657588 is to clarify what happens when a user clicks "cancel" on that dialog - namely, that Firefox will not send a client certificate. If the "remember this decision" box is checked, Firefox will remember that decision to not send a client certificate. If a user wants to use a different certificate, they'll have to delete the saved decision in the certificate manager.