So is there actually any reasons for having competitive open-source full browser stacks? I believe licensing reasons (the one behind FreeBSD and Linux) is not very important for browser world (Gecko and WebKit have very similar licensing terms). And I don't see much ideological reasons that can't be fixed by forking.
What else? Usually developers hate abandoning their work and switching to improve competitor's solutions — especially in the open source world. Developers like the feeling of doing important stuff and money. Market share of Firefox is still quite high (~25%) and Mozilla still receives money (company is non-profit, but developers are being paid I believe). Probably for this reasons the struggle will continue for some time.
However it is difficult to assume that there is something useful for web platform in this struggle. Every new web standard feature requires independent implementation from two different open-source projects and the whole platform adoption process goes as fast as the slowest team goes.