Newer versions of Firefox don't allow you to install add-ons that are unverified. You can bypass this, but I recommend doing it if you know for sure the add-on you're trying to install is safe to use.
Open Firefox, then type about:config. Accept the warning and type in the search box XPI. Look for xpinstall.signatures.required and change its value to false. Restart the browser, and the add-on can be enabled again.
