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I heard that in Firefox 55 a new feature was introduced as a part of the Tor-Uplift project which added a feature to Firefox called "First-Party Isolation" (FPI) which in Tor is known as "Cross-Origin Identifier Unlinkability".

As described here this feature does the following:

FPI works by separating cookies on a per-domain basis. This is important because most online advertisers drop a cookie on the user's computer for each site the user visits and the advertisers loads an ad.

With FPI enabled, the ad tracker won't be able to see all the cookies it dropped on that user's PC, but only the cookie created for the domain the user is currently viewing.

This will force the ad tracker to create a new user profile for each site the user visits and the advertiser won't be able to aggregate these cookies and the user's browsing history into one big fat profile.

I understand that this feature is still present in Firefox 57 (the current latest version at the time of writing). So how do I get this feature given that it is not enabled by default and not even listed in the official release notes (a serious amount of digging had to be done to even find it)?

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  • Given that this feature wasn't announced anywhere official I decided to make this Q&A to raise awareness of it. If you've seen it before it's because I first posted it on Information Security but then deleted it from there and reposted it here as a moderator suggested given that, although not a bad Q&A, it apparently wasn't on-topic there because it was more about configuration than the actual Security-side of things and thus it was closed.
    – user424955
    Nov 25, 2017 at 16:41

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This feature is not enabled by default because it can cause some user login persistence issues, however if you do want to enable yourself you can do so through the two following means:

1. By adding this dedicated (maintained by a third-party individual) add-on: First Party Isolation

This will add a fish bowl icon to the UI which can be clicked on if you want to temporarily (for 5 minutes) disable FPI:

BleepingComputers' screenshot showing fish bowl

If you change your mind and decide that you don't want FPI after you have added the add-on, then simply either disabling or removing the add-on will suffice in disabling FPI.

2. Or by tweaking the settings in about:config:

  • To enable FPI set privacy.firstparty.isolate to true.
  • If you are having problems logging into any websites due to FPI then you can set privacy.firstparty.isolate.restrict_opener_access to false and thus lower some of the isolation rules.

Original information source: BleepingComputers: Another Tor Browser Feature Makes It Into Firefox: First-Party Isolation

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