Matt at Allpeers posted about the pros and cons of Mozilla switching from Gecko to Webkit.
This is something that people in Mozilla have discussed, multiple times, because the pros are quite clear. We get feedback from many channels that Webkit is cleaner and a lot easier to hack on, and I believe that; its code footprint is also clearly better. Beyond that, open source Web browsers and the Web itself could make progress much faster if we were pulling together on the same code base instead of doing everything twice. It would also reduce compatibility pain for Web developers.
The cons are also quite clear, however. We really really really don't want to leave XUL developers --- that is, developers of Firefox addons and XULrunner applications, not to mention our own Firefox and Thunderbird developers --- in the lurch. So XUL and attendant technologies (XBL(2), some XPCOM glue) would have to be ported. Generally any Gecko features that Webkit doesn't have would have to be ported. Considerably bug-compatibility would be inevitably lost.
It's not clear how project management would work. Apple probably doesn't want all that stuff in their tree. Anyway, it would be unsafe for the Web to put all our eggs in their basket.
Some people argue that having two open source Web engines is good because it avoids the problems of monoculture. It can also help in standards bodies to have multiple interoperating implementations of a feature. There's some truth to it, although I think I'd rather have one awesome open source implementation than two less awesome ones. (I think in the long run, a single dominant open source system would be a healthier, more monopoly-resistant software infrastructure than several competing implementations unified via fractious standards bodies. I also think that the split between KDE and GNOME is about the worst thing that ever happened to the free software world.)
The biggest problem, however, is getting from here to there. Mozilla can't leave the market for a few years to attempt a risky transition plan. That would be bad for us and bad for the Web. We have to keep improving Gecko to get maximum leverage from our existing market share, mindshare, and developer share. Also, as I suggested in my second-to-last post, Webkit is not the epitome of engines. It would suck to spend all our resources getting there and then discover we should have gone in another direction.
None of this rules out Mozilla --- or a third party --- doing a XUL-on-Webkit project on the side! I actually think it could be terrific territory for a startup ... far more worthwhile than yet another buzzy social networking site. Or, dare I say it, Flock.
Lastly, let me deflect the charge of "Not Invented Here"ism by stating for the record that I have no great love for Gecko. I did not invent it, I think it has major flaws, and I would be most happy if it vanished and was replaced with something superior in every way. For the forseeable future, that's not an option.