If you want to run a custom AMO I'm assuming you're in a corporate environment or something like that where you can control what browser gets installed on people's machines.
https://addons.mozilla.org is an integral part of Firefox, if you set it up with an alternative you're effectively making your own fork.
It's not an integral part of Firefox, though. You can install add-ons without it by just clicking a link on any page that leads to an XPI, same as how AMO behaves.
And no, I'm not in a corporate environment. I'm talking about decentralization.
You can, AMO is open source: https://github.com/mozilla/olympia
Run your own instance and make your own builds of Firefox that point to it and you're good.