There are many many shallow forks of Firefox, Chromium or Webkit out there. Some prominent examples are SeaMonkey, Pale Moon, Basilisk, QuteBrowser, Falkon, Gnome Web, IceCat, Ungoogled-Chromium.

They're all superficial forks though. Without progress on the core engine etc upstream, they won't be able to keep up for long. On the other hand, there is little reason to think any of the three major engine efforts (Gecko, Webkit, Blink) are going to stumble any time soon.