As I would trust GrapheneOS for doing a insane job based on Android, if they make their "own" version of Chromium, I don't have much doubts that they know what they talking about on Chromium/Firefox.
Also looking at their forum, they answer lot of questions I can barely understand
As far as I understand, you kinda choose better security using Chromium or better privacy using Firefox with addons/forks
Lot of ressources say Brave is trash compared to Ungoogled, but Brave still adds stuff
Complicated, GrapheneOS recommends Vanadium or at least Brave on mobile