Waterfox is based on esr, so quite outdated. Just use librewolf and some css. You have firefox-one that will make it look pretty and similar to zen. Zen is no good if you care about privacy.

librewolf

I will take a look, it looks interesting. Thank you very much

I would suggest Debian. Rock solid and it usually has all you need. If you need newer software on it you can allways use backports and there is also a really cool project called distroboc that will let you run pretty much any application in a container.

I gave up on firefox 1 year ago and went to the dark side with Brave. I am really happy with it even tho part of it is closed source.