Zen is a new and interesting browser. This is a full guide to it.
To install it, just go to zen-browser.app and install for your platform. After that, open it and complete the onboarding process.
Right after you install Zen Browser, you should probably log into a Firefox account. If you already have one, then your data should be imported, and if you don't, you'll still get syncing for bookmarks and tabs.
Zen comes with a pretty good user.js file, so you shouldn't have to edit that, but in the settings, there are things you should change for privacy.