Good question, so far we have been building on top of chromium release that Google Chrome is based on.
Yes, I'm acutely aware of exactly how much compute pulling off such a stunt requires; what I'm wondering is whether you are aware of exactly how much RCE risk you're running by squatting on someone else's C++ codebase that ships what feels like a vuln-a-week from one of the best funded security research teams in the world
It's also not a rando library that changes 3 times a year and who fucking cares if it has vulns: this is one of the biggest attack surfaces known to mankind given what it does for the user
Merely as a "for comparison," rebuilding Firefox takes about an hour on a developer class workstation, which IMHO sure would make tracking upstream a lot less expensive