I'll see if I can export all modified flags but generally skim through about:config for references to "reporting", "telemetry", "pocket", "browser.ml".
On desktops I also like to enable UI density == 1.
In the settings GUI I generally disable sponsored links, autoplay stuff, studies and recommendations.
prior to anything, I install ublock origin and enable its use in private browsing.
Edit: if you use regular firefox for android (not beta, nor nightly), you can access about:config via chrome://geckoview/content/config.xhtml