There's a fix or you can update firefox.
What are my options if I want to use an unsigned add-on? (advanced users)
Firefox Extended Support Release (ESR), Firefox Developer Edition and Nightly versions of Firefox will allow you to override the setting to enforce the extension signing requirement, by changing the preference xpinstall.signatures.required to false in the Firefox Configuration Editor (about:config page). To override the language pack signing requirement, you would set the preference extensions.langpacks.signatures.required to false. There are also special unbranded versions of Firefox that allow this override. See the MozillaWiki article, Add-ons/Extension Signing for more information.
about:config in address bar,
then xpinstall.signatures.required in search field,
then right-click "xpinstall.signatures.required" value to toggle value from true to false