Firefox

That is blatantly false. Don't even waste your time.

I have this in user.js:

// settings user-test-programm
user_pref("app.shield.optoutstudies.enabled", false);

// dont use me as guinea pig
user_pref("app.normandy.enabled", false);
user_pref("app.normandy.optoutstudies.enabled", false);
user_pref("messaging-system.rsexperimentloader.enabled", false);

// side-loading of telemetry-extension
user_pref("extensions.systemAddon.update.enabled", false);

// disable Mozillas new tracking aggragation thingy
user_pref("dom.private-attribution.submission.enabled", false,);

// almost only for tracking useful
user_pref("beacon.enabled", false); // so webpage can send (tracking) data before you close tab
user_pref("browser.send_pings", false); // hyperlink auditing (click-tracking)

Note: the last two are more nuanced.

Argument for beacon is that webpages will use a more intrusive way with noticeable delay to upload data on tab close. I personally prefer that, as a warning, but never saw one after years.

Argument for send_pings is, that trackers will use more mean and stealthier ways to track you, if they don't have that interface (same as in private-attribution). I do know however, that companies who track you have high greed and low morale to begin with, and use all they can get to generate more money.

Just fire up Wireshark and inspect what Firefox calls, a lot of calling home and even if you change all the settings and config parameters to something sane it will still contact a 3rd party analytics company. Mozilla also acquired an ad analytics company recently for some reason.