Earlier quoted context omitted.
Google Analytics javascript runs on your domain and the cookies it uses are on your domain. So it should count as "first party" here.
There is no creation of cookies in the snippet you have to copy/paste on your website. So if some cookies are created they are not first party.
Inside the script there are calls to cookie-dropping javascript functions. The browser makes no distinction between javascript loaded via a script tag from your domain, another domain, or simply inline, so any cookies it leaves or reads are "first party".If they set cookies via HTTP on the request for www.google-analytics.com/ga.js, those would be "third party", and because google-analytics.com isn't a site anyone visits, the new firefox policy would reject them.