For those of you who want to keep using this and any other add-on that has not been "verified" yet by Mozilla, here's what Mozilla tells you to do, and I can confirm that it works. I'm on Firefox Developer Edition.

"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. "