Yeah the option is called Waterfox, Palemoon, or even Vivaldi.

Vivaldi is not open source. Not quite an option.

https://vivaldi.com/source/

I think the UI code is not open source (so you can't build the browser yourself).

https://vivaldi.com/blog/technology/why-isnt-vivaldi-browser...

Wait, what? Vivaldi is open source? Now I am confused and really not sure what was the reason I ignored it for so long. Was there something iffy with Linux desktop integration?

It is not open source. Some of the backend is.

Quite surprised at Vivaldi. Considered that as Opera spiritual successor including any possible feature, will've been one of the first browsers adding AI.