I’ve switched from Librewolf to Zen for a few months now, and it’s been great so far. There is all the fun features like the essentials tab, and the tab groups. Also Zen Mods is super cool. It is like a repo of css mods for zen.

There was some controversy about the debug thing, but as others pointed out, it happened during the alpha phase, when the development team is still new to the project.

For hardening, you can use Arkenfox, Betterfox, or Phoenix. All of these work on Zen (I’ve tried them all). Librewolf is based on Arkenfox, so if you use it, you’ll get most of Librewolf’s privacy features. I recommend Arkenfox. Personally, I believe Betterfox is a balance take between privacy and usability, while Phoenix is the most extreme option. Arkenfox offers stronger privacy features than Betterfox but is not as extreme as Phoenix.

Edits: also don’t forget uBlock :)