Why does it appear?

As part of Safe Browsing, Web browsers ping Google periodically for updated lists of potentially dangerous sites. When they do, Google puts a cookie on the user’s machine. Google says the cookie helps it keep its system stable and monitor for attacks.

Source: The Google Cookie That Seems to Come Out of Nowhere

Why does it keep reappearing even after I delete it?

Because you need to disable Safe Browsing AND because Firefox Cookie Manager can't delete it, even if it appears to (bug #1026538).

Why is it dangerous?

As this cookie contains a unique ID number, it has been used by the NSA to track people under suspicion. Source: NSA uses Google cookies to pinpoint targets for hacking, Washington Post

Also, it means Google can track you better since this unique ID is persistent even after you close Firefox.

Security often means less privacy. You can avoid sending all your browsing history to Google: Use an up-to-date browser and modules, disable uneeded browser modules, don't install apps/modules from untrusted sources and avoid phishing attempts by checking the website domain and HTTPS certificate.

How to really get rid of it?

Disable Safe Browsing: 1.1 Uncheck "Block reported attack sites" under Firefox Preferences > Security tab 1.2. Uncheck "Block reported web forgeries" under Firefox Preferences > Security tab THEN manually delete the existing cookie with sqlite3 (as long as bug #1026538 is open) 2.1. Find your Firefox cookie database within your Firefox profile folder: Firefox menu > Help button > Troubleshooting Information > Application Basics section > Profile folder line > Open Directory button > File name is cookies.sqlite or (Ubuntu) find ~/.mozilla/firefox -name cookies.sqlite 2.2 Install sqlite3: Download or (Linux) sudo apt-get install sqlite3 2.3 From command prompt: sqlite3 path-to-cookies.sqlite 2.4 DELETE FROM moz_cookies WHERE baseDomain = "google.com";

Now you can check that the PREF cookie doesn't reappear at Firefox launch in Firefox Cookie Manager. It should not reappear as long as you don't re-enable Safe Browsing and if you have configured Firefox to delete cookies after exit.

Recommeded tools to limit tracking (except PREF cookie...): Cookie AutoDelete