By default, Firefox requires that all of your Add-ons have been digitally signed by Mozilla before enabling them. If you need to enable an Add-on that has not been signed, use the following steps to disable the requirement.

  • Navigate to about:config in the address bar.
  • Search for xpinstall.signatures.required.
  • If it exists, set it to false.
  • If it does not exist, right-click to create a new Boolean preference, name it xpinstall.signatures.required and set it to false.
  • Restart Firefox

After restarting the browser, the Add-on should be enabled. It will also display a warning about it not being verified.

(tested on Firefox 47.0.1)