Go and have a look? https://github.com/brave/brave-browser

Being open-source doesn't automatically make you secure or reputable. Especially considering the open-source ecosystem in particular is a big target for exploitation right now. And auditing a software project of this size by its source code alone is no small feat.

it is also the most private and secure, open-source, mainstream* Chromium browser

"Mainstream chromium browser" is doing most if not all the heavy lifting there. Fair enough if that's what you're after, but mixing "private and secure, open-source" in feels disingenuous.

That said, I primarily use Vivaldi because of its customizability and added features, something Firefox seems to reduce with every new version.

Last time I played with either Vivaldi or Brave you had to literally monkey patch the source code in order to customize things further than what the extension SDK allowed you to. You could do the same thing with Firefox, except they make it slightly harder because much of the source code is shipped in archives.

That said it's been years, maybe this can now be done purely through the extension SDK? It'd be news to me.