I believe a lot of info I got was from this video but it’s been a while so I’m not too sure: https://youtu.be/ugnOM2mzgNU
Also yea Firefox sends a lot of telemetry data and stuff, even if you disable the option in the menu. You have to go to the developer mode to remove all of it. Check "hardened Firefox". If there is an hardened Firefox, then there is a non-hardened Firefox.
And then there are all the contracts and calls to Google's server, for example for geolocation and stuff
And if you want the ultimate proof, everything is in their privacy policy https://www.mozilla.org/fr/privacy/firefox/ - just see how much data they collect, use and share, for better or for worse.