We enabled the feature in Nightly in Bug 1877432 in "dry run" mode, which means the mechanism would classify trackers but not purge them. This bug is for fully enabling the BTP feature to also purge tracker cookies & storage periodically.

Please request a nightly-only release note once it has landed, thanks!
https://wiki.mozilla.org/Release_Management/Release_Notes_Nomination

https://hg.mozilla.org/mozilla-central/rev/6fb3090e860b

Release Note Request (optional, but appreciated)
[Why is this notable]:
We're trialing a new anti-tracking feature in Nightly: Bounce Tracking Protection. It's similar to the existing Cookie Purging mechanism in Firefox, but instead of tracker lists it uses heuristics for classification of bounce trackers.
While the feature avoids purging cookies for sites that the user has interacted with recently, there may still be false positives that can lead to users getting logged out or loose site data. For troubleshooting or bug reports users/devs can enable logging e.g. via about:logging using the following logger: BounceTrackingProtection:3.

[Affects Firefox for Android]:
Since this is a feature built into Gecko we are also enabling it in Fenix Nightly.

[Suggested wording]:
We're enabling a new anti-tracking feature in Nightly: Bounce Tracking Protection. This protection detects bounce trackers based on redirect behaviour and periodically purges their cookies & site data to prevent tracking.

[Links (documentation, blog post, etc)]:
Spec: https://privacycg.github.io/nav-tracking-mitigations/#bounce-tracking-mitigations
Intent to prototype: https://groups.google.com/a/mozilla.org/g/dev-platform/c/M6erM0SjPTM
What is redirect / bounce tracking: https://blog.mozilla.org/security/2020/08/04/firefox-79-includes-protections-against-redirect-tracking/

Note added to https://www.mozilla.org/en-US/firefox/127.0a1/releasenotes/ Thanks!

Starting with Firefox 127, we're enabling a new anti-tracking feature in Nightly: Bounce Tracking Protection. This protection detects bounce trackers based on redirect behavior and periodically purges their cookies & site data to prevent tracking.