You can learn about how to stop Firefox from making connections using this resource from Mozilla:
Mozilla often appends Urchin Tracking Module parameters at the end of their URLs, so if you want to remove them, along with other URL parameters, here is a guide I wrote for doing so with uBlock Origin:
about:preferences does not contain every modifiable configuration value. Nearly all of them are visible in about:config, with some described in about:policies, which are specific to Firefox ESR. You can save yourself the trouble of manually configuring any of these every new install by using a user.js and policies.json (Firefox ESR only) instead, with the former within your Firefox profile’s root directory (Firefox ESR on PureOS: /home/purism/.mozilla/firefox/${PROFILE}.default-esr), and the latter within Firefox’s installation directory (PureOS: /usr/lib/firefox-esr/distribution), which will act as persistent configuration files. If you want multiple Firefox profiles for various use cases, you can access about:profiles to manage them.