That is no longer available with the current Firefox.

There are some options for advanced users toward the end of this article: Add-on signing in Firefox.

belloobscuri said

I'm really frustrated with firefoxs ridiculous policy on add-ons. For no-name companies/developers/whatever, I would understand - but I've got two add-ons from very well known sources that "could not be verified" and so are disabled. However, the real trouble is when I change the about:config - xpinstall.signatures.required to false. It STILL won't let me enable them. What do?

The about:config still works for you.

The signing of Extensions change already happened before Firefox 48.0 and the xpinstall.signatures.required preference was just temporary for those who discovered it and was never intended to be long term.

Extensions hosted by the author at addons.mozilla.org should be signed unless really old.