Mozilla has announced that it plans to block third party cookies by default in the Beta version of Firefox in a blog post. The reason the company has given for this is, “to collect and analyze data on the effect of blocking some third-party cookies.”

The basic idea behind the plan to block third-party cookies is that if you have not visited a site and it wants to put a cookie on your computer, there is a high chance that you do not actually want any cookies from the website. However, according to Mozilla, two problems pop up – that of false positives and false negatives.

Firefox 14 released

Not blocking third-party cookies by default just yet

The problems with false positives arise when, for example, you visit a website named, which embeds cookie-setting content from a website named After the cookie-blocking patch from Firefox is out, the browser will allow cookies from, but will block because you never visited the site, even though both websites belong to the same company.

False negatives are a problem when you visit a website, and it still insists on putting cookies into your computer, whether you like it or not. An example of this is when you accidentally click on an ad, or when a website that you may trust starts setting third-party cookies that you do not want.

Mozilla wants to ship a patch with the setting being turned on by default, but it still needs some refinement. According to Mozilla, “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.”

Tags: , , , , , , , , , ,