"open" does not mean "fun"
The source for Firefox is getting to be a bit of a sprawling disaster area. Find me some people willing to hack on its source, especially given its kinda frustrating license annoyances, and I'd be happy to join them in promoting an improved fork.
It would probably be easier to just write a better browser from scratch, apart from the rendering engine itself, though. I think it would also be easier to just maintain an extension system patch for Chromium.