|| I'm also a little surprised there hasn't been an attempt to launch a completely new Firefox from the ground up.
I (hopefully) think this is what Servo is and will end up being.
Has no legacy code, and is tiny in comparison to Gecko. It's OS support is fairly modern, and has no interest in supporting Windows XP.
It's also written in such a way that allows them to be more multi-threaded and more concurrent than traditional rendering engines.
At some point you have to draw a line in the sand and start again, and I hope Servo is that.
You can check out https://github.com/browserhtml/browserhtml which is servo running standalone