It seems like the Firefox team intends to only have what maximises the privacy security protections, and that this extra functionality (hereafter referred to as SameCon, versus the current DefCon) is ideally implemented as an add-on. I'd imagine they're thinking that if a user wants SameCont, that user can just add it as an add-on

I don't think this is the ideal solution, because I'd imagine there are a population of users who just wouldn't consider the possibility that SameCon could Be an add-on. Especially if the community is split roughly down the middle.

I think yours is the ideal solution, making SameCon a configurable option, but having the default option be DefCon. That way, the privacy and security protections are the Default behavior, but a user has the option to change it Built In