Projecting political views onto browser users by default is a big one (new tab). Another is the limiting of user control by not allowing them to install third-party extensions. They also enable telemetry by default, and that not all telemetry can be disabled without going into about:config (i.e the settings in the preference page do not disable telemetry about telemetry itself; the browser can and will still send some telemetry even when telemetry is set as disabled).

I (and many others) use Firefox because we need a trustworthy browser, but issues like these take away from that trustworthiness.