I would like to refresh that topic because I need some help as well. I’m using Firefox 57.0.1 and addons aren’t working. They are Legacy and unsigned. Some explanation first.
Legacy Addons are XPI old type which Mozilla no longer provide. To enable them configuration must be changed so about:config; extensions.legacy.enabled = true.
Some time ago Mozilla introduced digital signing for addons. Those not signed will not work. To override about:config; xpistall.signatures.required = false.
That doesn’t work in Firefox 57.0.1. There is no Enable button next to each extension so they are listed as good to go with warning to be careful but can’t turn them on.