Current Firefox version’s Private browsing mode has tracking protection enabled by default (but not in normal mode or non-private windows), recently, in Nightly build, Mozilla has removed the Polaris preference from about:config and enabled UI to toggle tracking protection by default in non-private windows.
Till now, to enable TP in normal mode, user need to visit about:config and change privacy.trackingprotection.enabled preference value to ‘true (this works on all channels)’, this no longer required in latest Nightly.
Nightly users can now turn on tracking Protection in normal mode by selecting Always in Privacy of the Options under ‘Use Tracking Protection‘.
You can confirm tp is enabled or not in NM by visiting the Control Center (that shows site privacy and security controls in one place in the address bar), which displays the tracking Protection badge and information about tracking elements detected on the page you’re on.
Remember, this is an experiment only, may not be shipped to release channel users.