Helium has uBlock Origin built in. It is the most lightweight yet functional browser I've found. That being said, there's only so much you can do with browsers to slim it down, and I only notice the speed difference in truly ancient hardware. It's also basic in terms of settings but that can be seen as a plus. Stock Firefox or even worse, Chrome or Edge feel like sensory overload to me if I go back. It annoys me that it is only available as an AppImage on Linux.

Personally I use Brave Origin on most devices, so I don't mind Chromium, it's actually significantly better and more secure/private on Android. On any desktop Firefox based stuff is just as good but I use the same browser for consistency and bookmark synching without an account. Brave has more active privacy features than Helium too, though Helium does remove telemetry which is the only way I'd use Chromium, so Helium is a perfectly reasonable choice.

I wasn't aware of any controversy with Zen. It may have telemetry options but they can be turned off just like Firefox. You're trading marginally slower updates for a better Ui, that's the only difference from Firefox really. The maximizing of space is really cozy and macOS-y but it is more customizable if you want something a bit flashier too. It is as reasonable choice as Firefox more or less, meaning a very good one that is sadly trending down.