What does the Google Analytics Opt-out Browser Add-on do?

I recently tried to opt-out of Google Analytics tracking on a website using my secondary browser (no other tracking blocking active) and, expecting to be given a cookie setting, was surprised to be presented with the Google Analytics Opt-out Browser Add-on.

This seems an incredibly strange and heavyweight solution to stop tracking (which should be opt-in, but that's another story). What exactly does this add-on do? Does it introduce further privacy issues?

What exactly does this add-on do?

The browser add-on works by running a function on every web page to set a variable turning Google Analytics scripts into no-ops. The code injected by the extension can vary slightly depending on which browser the extension is written for and is similar to the one below:

/* Copyright 2017 Google Inc. All Rights Reserved. http://tools.google.com/dlpage/gaoptout/intl/en/eula_text.html */
(function() {
    var a = document.createElement("script");
    a.type = "text/javascript";
    a.id = "__gaOptOutExtension";
    a.innerText = 'window["_gaUserPrefs"] = { ioo : function() { return true; } }';
    document.documentElement.insertBefore(a, document.documentElement.firstChild);
})()

Does it introduce further privacy issues?

According to Giorgio Maone, yes. He states a website could reset the _gaUserPrefs variable to bypass the opt-out and that the presence of the variable can increase the uniqueness of the user agent's fingerprint.

Note that there is nothing inherently "heavy" about a browser add-on. The one distributed by Google is rather lightweight.