Internet Explorer but hear me out before you cry:

The current browser ecosystem has more or less two engines: WebKit based browsers and Firefox. We have basically WebKit monoculture. If you'd find a fundamental flaw in WebKit, you'd be able to break most browsers. If Internet Explorer would be open source we would have the chance to get a third engine running. Browser engines aren't trivial and IE also had some good sides after all these years.

I guess if IE would become FOSS this would bring some fresh air to the ecosystem.