[Sylos]

That's because Firefox Focus doesn't actually have an ad blocker. What it has is a tracking blocker. It just so happens that almost all ads have trackers in them, that's why it still blocks most ads.

The ad results on web search engines generally don't have trackers in them, that's why they for example don't get blocked. Also, webpages with privacy-conscious users tend to have privacy respecting ads and webpages that are typically opened in Private Browsing tend to at least have privacy-respecting ads as backup, since Firefox's Private Browsing has this same tracking blocker.