Because Mozilla takes a metric shitload of your data via fakespot such as (but not limited to)

Internet or other electronic network activity (e.g., browsing history, search history, information regarding an individual’s interaction with an internet website, application, or advertisement, and online viewing activities)

https://www.fakespot.com/privacy-policy

And then sells it to advertisers

apt remove firefox (or via pacman, windows settings etc)

Otherwise should be a bunch of flags you can set in about:config

Lol who the f trusts Mozilla nowadays?

https://www.fakespot.com/privacy-policy

Internet or other electronic network activity (e.g., browsing history, search history, information regarding an individual’s interaction with an internet website, application, or advertisement, and online viewing activities)

Category of Third Parties to Whom Personal Information is Sold and/or Shared: Advertising partners, Service providers

Just a snippet of the privacy policy. There’s other bad stuff too like location tracking. It’s also all ran through Google analytics.

So much for a privacy respecting Mozilla