The current version of Firefox’s private browsing mode has tracking protection enabled by default (but not in normal mode or non-private windows), recently in the nightly build Mozilla removed the Polaris preference from about:config and enabled the user to switch tracking protection by default on non-private windows.
So far, to enable TP on normal modethe user needs to visit about:config and change privacy.trackingprotection.enabled preference value to ‘true (this works on all channels)’, this is no longer needed in later Every night.
Night users can now enable tracking protection in normal mode by selecting Ever inside Privacy of Options in ‘Use tracking protection‘.
You can confirm whether or not tp is enabled in NM by visiting the Control Center (which shows the site’s privacy and security controls in one place in the address bar), which displays the tracking protection badge and information about elements tracking detected on the page you are on.
Remember, this is just an experiment, it cannot be sent to release channel users.