It’s just the engine, but it’s supposed to be a much more modern and smaller engine, so writing a new browser on top of it would be much easier than using gecko is. But you’re right, it’s not a browser. There is Verso as a prototype browser, but it’s far from functional.

Ublock is added by default to librewolf, unless you installed it in an unconventional way.

When it comes to zen I haven’t really seen a compelling reason for it. I care about privacy more than functionality, but I don’t really want to go about hardening a whole new browser.

I have found that using PWAs with librewolf as the engine is very in lign with my mental image of how to run things.