In order to install an extension as a normal add-on into Release or Beta versions of Firefox greater than, or equal to, version 48, the extension must be signed by Mozilla. An extension is signed by submitting it to AMO. Once it is signed, the extension can be installed on any version of Firefox which it supports. For Firefox versions prior to version 43, extensions were not required to be signed by Mozilla. Types of add-ons other than extensions are not required to be signed by Mozilla.
You can install unsigned extensions as normal add-ons into other versions of Firefox (e.g. Firefox Developer Edition, Firefox Nightly, Unbranded Beta, or Unbranded Release) by setting
about:config. Setting this option was also effective in Release and Beta Firefox versions 43–48. Setting this option is not effective in Release and Beta versions of Firefox from version 48 onward.
The need to be able to install unsigned add-ons during add-on development has been greatly reduced by the availability of installing extensions as Temporary Add-ons. Temporary Add-ons do not need to be signed, and can be loaded into any current version of Firefox. As the name implies, the primary drawback of Temporary Add-ons is that they are temporary. They must be re-installed any time Firefox is restarted. However, there are use cases where it is preferable to install an add-on as a normal, but unsigned, add-on instead of as a Temporary Add-on. An add-on should be installed as an unsigned add-on if there is need for the add-on to remain installed even after Firefox is restarted. This could be desirable for a variety of reasons, including: longer term use testing, or to test how the add-on functions when Firefox starts up.
if you want to reproduce, please indicate the source:
Getting started with firefox-addon – Installing unsigned add-ons - CodeDay