I stumbled across this by accident. Was trying to find exactly what Mozilla will use to "identify you" on their new Firefox Account login. Really didn't find anything except marketing hype. Fact: lots of Fx addons don't work on Mozilla sites. Fact: Mozilla sites allow Google Analytics (I'll use GA, ga or "_ga") to set a 1st party cookie. https://www.mozilla.org/en-US/privacy/websites/ Mozilla says: Sign into Google acct? Meaning, you must have one. To opt out of Google gathering data on you? Didn't know this addon existed, developed by Google. I'd think the reason for requiring creating an acct & logging in to change some settings is fairly obvious. On to the question if GA cookie set on Mozilla sites really can be accessed by GA on other sites, as this site / blogger claimed. https://www.upbuild.io/blog/firefox-google-analytics-data/ Meaning, the GA 1st party cookie isn't blocked (OK). [author, Mike Arnesen was using Fx 67, which may function differently than v71). Mike says, in Firefox, hit Shift + F9 to bring up the Storage Inspector. ... see any cookies,...( He shows how the UNblocked 1st party _ga cookie is set (if a site allows this in their settings). https://i0.wp.com/www.upbuild.io/wp-content/uploads/2019/06/ga-cookie-verification-in-firefox-1.png?ssl=1 Now the interesting part, if still true. Mike says: Again, this was supposed to be a 1st party cookie. I understand this fairly well - up to a point. What I don't know is whether the "default" Fx 67 settings he used did block GA's 3rd party but not their 1st party cookies. What do Wilder's resident gurus think of this? If you don't have everything set to block and google sets 1st party cookies on mozilla sites, is it ever possible for Google to read the cookie across multiple sites? We know Google gets preferential treatment. In Privacy & Security under enhanced tracking, I have "Custom" checked w/ "cookies from unvisited sites; "Tracking content - In all windows" checked. The other 2 options are checked. Also, in about:config, "privacy.firstparty.isolate" = true. I think that was default in Fx 67. With those settings (which will break some features on some sites), I see no _ga cookie set - logging into //support.mozilla.org. I had to uncheck "Tracking content" under Custom security section. Then 3 GA cookies were set. Before going to other sites, I re-checked under Custom security, Tracking content [NOTE: when that is unchecked / re-checked, it defaults to "only in private windows" (private browsing - so be mindful). Went to Amazon, Google.com then back to Mozilla (so I could see the original GA cookies set there, to see if "Last accessed" time had changed (I didn't re-load Mozilla). It hadn't, but I'm not sure what would've happened if I didn't re-check the Security & Privacy > Custom settings to "full lock down" before visiting the other sites. But I noticed something interesting in one of mozilla.org's source code: It said "Github secret," with a long, random alpha-numeric string. I think it was when I logged onto Mozilla's site, but couldn't find it again.