[freethinky]

At least until 82 I (have to try if it still works), I used:

about:config xpinstall.signatures.required -> false

    zip -0 mywebext.xpi manifest.json *.js *.png *.html

about:addons -> install xpi

[AlexITC]

That worked for me only after including this snippet on the manifest:

  "browser_specific_settings": {
    "gecko": {
      "id": "[e-mail]"
    }
  }