Mozilla is engaged in a broad, deep conversation about Internet privacy. We believe in putting users in control of their online experience, and we want a healthy, thriving web ecosystem — we do not see a contradiction. However, sometimes a crucial experiment is required to prove it.
To this end, we are testing a patch from Jonathan Mayer. Jonathan’s patch matches how Safari has worked for years, and does the following:
- Allows cookies from sites you have already visited.
- Blocks cookies from sites you have not visited yet.
The idea is that if you have not visited a site (including the one to which you are navigating currently) and it wants to put a cookie on your computer, the site is likely not one you have heard of or have any relationship with. But this is only likely, not always true. Two problems arise:
False positives. For example, say you visit a site named foo.com, which embeds cookie-setting content from a site named foocdn.com. With the patch, Firefox sets cookies from foo.com because you visited it, yet blocks cookies from foocdn.com because you never visited foocdn.com directly, even though there is actually just one company behind both sites.
False negatives. Meanwhile, in the other direction, just because you visit a site once does not mean you are ok with it tracking you all over the Internet on unrelated sites, forever more. Suppose you click on an ad by accident, for example. Or a site you trust directly starts setting third-party cookies you do not want.
Our challenge is to find a way to address these sorts of cases. We are looking for more granularity than deciding automatically and exclusively based upon whether you visit a site or not, although that is often a good place to start the decision process.
We plan to ship an evolution of the patch “on” by default, but we want to make refinements first. To make sure we get this right we need more data. Our next engineering task is to add privacy-preserving code to measure how the patch affects real websites. We will also ask some of our Aurora and Beta users to opt-in to a study with deeper data collection.
There are many conflicting claims about how this patch will affect the Internet. Why debate in theory what we can measure in practice? We are going to find out more and adjust course as needed. This is the essence of the release test cycle.
On Tuesday we did two things:
- The patch has progressed to the Beta release channel for Firefox 22, but it is not “on” by default there. This allows more people to test the patch via Firefox’s “preferences” (AKA “options”) user interface, and avoids an abrupt change for site owners while we work on handling the hard cases.
- The patch remains in the Aurora channel for Firefox, where it is “on” by default. This gives the patch better ongoing test coverage and facilitates A/B testing.
We have heard important feedback from concerned site owners. We are always committed to user privacy, and remain committed to shipping a version of the patch that is “on” by default. We are mindful that this is an important change; we always knew it would take a little longer than most patches as we put it through its paces.
For those who read this as Mozilla softening our stance on protecting privacy and putting users first, in a word: no. False positives break sites that users intentionally visit. (Fortunately, we haven’t seen too many such problems, but greater testing scale is needed.) False negatives enable tracking where it is not wanted. The patch as-is needs more work.
We look forward to continued dialog with colleagues, contributors, fans, and detractors. We will update all of you within six weeks so you can understand our thinking and how we will proceed. Comments welcome.
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P.S. Cookies (name history) were originally intended to be ephemeral (Windows 3.1 had so little usable memory with its 64K memory segments that Netscape’s founders had no choice). At first, they held only session state that could be recovered from the server by logging in again.
(Remind me to tell the story some day of Montulli’s aborted “twinkies” idea from the Netscape 2 era. UPDATE: Lou has published a new blog post about cookies.)
How far we have come in the amazing, living system that is the Web! No one planned for what actually happened, but with more work on the cookie policy in Firefox and (I hope) other browsers, I believe that we can evolve to a better space.