Chris Beard, writing at the Mozilla blog:

Microsoft is officially giving up on an independent shared platform for the internet. By adopting Chromium, Microsoft hands over control of even more of online life to Google.

[…]

We compete with Google not because it’s a good business opportunity. We compete with Google because the health of the internet and online life depend on competition and choice.

There’s a minority of people who are concerned about Microsoft’s announcement. Everyone web developer I know who actually has to comply with browser standards is very happy to hear it though. In a perfect world, there would be one open source browser with 100% market share.