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Surprise, surprise. Mozilla slipped Firefox's built-in Tracking Protection feature into the Firefox 57 stable release next week.

Tracking Protection was integrated into Firefox back in 2014, and made available in release versions of the browser with the release of Firefox 42.

Tracking Protection, as the name implies already, is designed to block tracking scripts on the Internet. The feature is not a full-blown content blocker, but a feature that improves user privacy on the Internet by blocking trackers, usually those from advertising scripts.

A side-effect of this is that it will block some advertising scripts when enabled. A Mozilla study revealed back in 2015 that enabling Tracking Protection in Firefox decreases page load times by 44%.

Tracking Protection is enabled by default in private browsing mode, but no option was available in the Firefox preferences to enable it fully in the browser.

Source. Firefox 57: full Tracking Protection functionality included - gHacks Tech News