How PREF cookie (google.com) appears in firefox?

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?

  1. Because you need to disable Safe Browsing
  2. 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?

  1. 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

  2. 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: sqlite3path-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


It's used by the NSA and GCHQ to spy on people!

http://rt.com/usa/nsa-advertisers-cookies-track-browsers-034/


This cookie is coming from Google's safebrowsing api:

google.com/safebrowsing/downloads?client=....

This is a known issue in firefox and there is an open ticket here:

https://bugzilla.mozilla.org/show_bug.cgi?id=368255

EDIT To get rid of it, you can turn off the safebrowsing on the security tab (under Options), by un-checking the boxes to Block attacks and forgeries.


Google has updated their policies page to explain what types of cookies they use, specifically the PREF cookie.

But beforehand I will say that I can't explain where and how this cookie pops up in the browser. It seems to be done by firefox itself even if you don't use Google search, Google safebrowsing and block cookies for °.google.com

Google's policies page states that:

Preferences

... The PREF cookie may store your preferences and other information, in particular your preferred language (e.g. English), how many search results you wish to have shown per page (e.g. 10 or 20), and whether or not you wish to have Google’s SafeSearch filter turned on.

Advertising

... Google uses cookies, like the PREF cookie, to help personalize ads on Google properties, like Google Search, particularly when you aren’t signed in to a Google account. ...

Maybe it's part of the undisclosed contract between Mozilla and Google to set this unblockable zombie cookie. Who knows? :-)

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Firefox

Cookies

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