They call it "tracking protection." As a side effect, it blocks ads. I guess they wouldn't block ads that don't track you, but those seem to be rare.

Edit: Is my experience unusual? News websites that are normally encrusted with ads (e.g. using Firefox for Android without any add-ons) turn refreshingly ad-free in Firefox Preview.

You might get some accidental adblocking on some news sites through tracking protection, but with reader mode I probably wouldn't really notice.

I mostly use ublock origin to block ads in search, YouTube, Instagram, Facebook or Reddit. AFAIK none of those are blocked by the current Firefox tracking protection.

Ads on Google Search and YouTube aren't blocked. I didn't find any ads to test on Instagram or Facebook without logging in, and didn't feel like creating an account just to check out their ads. Ads on Reddit did get blocked. They're probably using third-party adtech, unlike the others.

I guess my browsing habits are unusual, since the only website among these I've visited since switching to Firefox Preview is Reddit.

why would you visit Reddit from a mobile browser? can I tell you about these things called "apps"?

Reddit tries to tell me about these things every time I visit, but I just don't see the point. Does the app improve anything in comparison to the website?

'cause it's a website?

It may block some ads but it's still lightyears behind Firefox for Android with uBlock Origin in my experience.