Firefox in its default settings is crap. However, it is still the best browser that exists in my opinion. The reason is that it has a lot of flexibility under the hood.
Type in about:config into the address bar and that is where all the settings can be changed. The amount of telemetry your browser sends to remote servers is astonishing. Type in ‘telemetry’ into the search within the about:config page. Outgoing telemetry can easily be illustrated by using an application such as Net Activity Viewer (linux only) or Netguard (Android). You can disable much of this.
You can customise the browser to be very secure using a file called user.js which is then placed into the user profile folder (desktop version). You can have multiple profiles with varying degrees of security that can be clicked open when you choose to so you can have multiple browser windows open all treated as separate sessions, so not sharing cookie data, etc.
However, Mozilla are going to be removing access to the about:config facility in an future version so we need even more choice unfortunately. This seems to be far and few between.
Even Brave Browser which many proclaim to be secure sends out loads of telemetry, or at least it did when I last experimented with it.
Right now Firefox secured using the settings within about:config is the best browser. Better than ungoogled Chromium.