This is going to be a hard truth, and it is not going to be popular either.
I just finished building my own copy of Mozilla Firefox. I disabled, everything, and I do mean, everything. When done, there were no user settings, not even the simple back, forward, and home button. I stripped Gecko down to its core components, so the only function was to render websites.
You cannot optimize Firefox further (I don't think). I gutted it like a fish, and I think pushing it further would have broken it