>But the issue here (AFAICT) is that Mozilla won't even let you sideload your own stuff on your browser.

There's no way to allow that that doesn't also allow crapware / malware installers from injecting stuff into Firefox.

You can install unsigned or foreign-signed addons, you simply need either the developer editino or you use a unbranded version of Firefox (ie, any Firefox binary that isn't branded for Mozilla). It's a compile option.