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This add-on enables the First Party isolation pref. Clicking the Fishbowl icon temporarily disables it.

This add-on enables First Party isolation. Clicking the Fishbowl icon or removing the add-on disables it again.

First Party Isolation, also known as Cross-Origin Identifier Unlinkability is a concept from the Tor Browser. The idea is to key every source of browser identification with the domain in the URL bar (the first party). This makes all access to identifiers distinct between usage in the website itself and through third-party. Think of it as blocking Third-party cookies, but more exhaustively. Here are Firefox's implementation details about First Party Isolation.