As for Firefox vs Brave, I would encourage you to check out the independent study by Leith, of Trinity College in Dublin. Leith found Brave to be in its own class as the "most private" browser tested (including Firefox): https://www.scss.tcd.ie/Doug.Leith/pubs/browser_privacy.pdf.
You could perform a similar test yourself. Setup Fiddler on your machine (installing the root cert to inspect HTTPS traffic), set aside %localappdata%/Mozilla and %appdata%/Mozilla, then launch Firefox. The traffic (without any user interaction) is a surprise to many. Compare with Brave (be sure to set aside %localappdata%/BraveSoftware beforehand).
The paper you linked to is limited in scope to the browser itself calling home with unique identifiers. The paper says nothing about what happens when the browser visits say Facebook.
In terms of the browser calling home and uniquely tracking you all by itself, the paper says Brave does a good job. But Brave still calls home for things and Brave can still collect IP addresses and browser fingerprints the same as any other site.