What Firefox Strict Tracking Protection Blocks

This article provides a comprehensive overview of the additional trackers and scripts blocked when switching Mozilla Firefox’s Enhanced Tracking Protection (ETP) from Standard to Strict mode. While Firefox's default Standard mode blocks basic cross-site cookies, social media trackers, cryptominers, and known fingerprinters, Strict mode significantly tightens security. Below, we examine the specific extra layers of defense activated in Strict mode, including blocking tracking content across all browsing sessions, halting suspected fingerprinters, and stripping tracking parameters from URLs.

Tracking Content in All Windows

In Standard mode, Firefox only blocks "tracking content" when you are using a Private Window. Switching to Strict mode blocks tracking content in all regular browsing windows as well.

Tracking content refers to hidden tracking scripts embedded within functional website elements, such as advertisements, videos, comment sections, and analytics tools. By utilizing the "Content" category of the Disconnect protection list, Strict mode stops scripts from companies like Google Analytics and various ad networks from loading at all. Because these scripts are often tied to how a webpage functions, blocking them in all windows increases privacy at the risk of occasionally breaking video players or login portals.

Suspected Fingerprinters

Digital fingerprinting creates a unique profile of you based on your computer's hardware, operating system, browser settings, installed fonts, and screen resolution.

While Standard mode blocks known fingerprinting scripts identified on Disconnect’s blocklist, Strict mode adds protection against suspected fingerprinters. It does this by actively restricting scripts from accessing specific Web APIs (such as Canvas, WebGL, and AudioContext) that are commonly abused to harvest your system's configuration data, even if the script is not yet on an official blocklist.

URL Tracking Parameters (Query Parameter Stripping)

Websites frequently attach unique tracking codes to the end of URLs to see where you came from and who shared the link with you. In Strict mode, Firefox activates a feature known as Query Parameter Stripping across all browsing sessions.

This feature automatically deletes known tracking tags from web addresses before the page loads. Examples of stripped scripts and tags include:

fbclid (Facebook Click Identifier)
gclid (Google Click Identifier)
mc_eid (Mailchimp Email Identifier)
msclkid (Microsoft Click Identifier)
oly_enc_id and oly_anon_id (Omeda Marketing)

While Standard mode blocks basic social media trackers, Strict mode aggressively blocks the functional scripts associated with third-party logins and embedded social content. Scripts that allow you to "Sign in with Facebook" or "Sign in with Google," as well as embedded X (formerly Twitter) feeds and Disqus comment sections, are completely blocked from executing until you manually grant them permission.

Network-Level Bounce Trackers

Bounce tracking occurs when you click a link and are momentarily redirected through an intermediate tracking server before landing on your final destination. This brief "bounce" allows the intermediate server to act as a first-party site and drop tracking scripts and cookies onto your browser. Strict mode enables enhanced Bounce Tracking Protection, which detects this redirect behavior and automatically purges the cookies and site data set by the bounce tracker.

SmartBlock Script Replacement

Because Strict mode blocks so many additional scripts, many websites would normally fail to load correctly. To counter this, enabling Strict mode also activates Firefox's "SmartBlock" feature. SmartBlock intercepts the blocked third-party tracking scripts and replaces them with safe, local "stand-in" scripts stored within the Firefox browser itself. This ensures that the website thinks the script has loaded, allowing the page to function without sending any of your data to the tracker.