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.

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.