Forking a big, fast moving, security critical piece of software like a browser is totally impractical unless you have a sizable company behind you to maintain it.

So what? Lots of things are impractical for individuals.

It's not just individuals. If people can't contribute to the project it is hard to build or maintain knowledge about that technology so another party can use it at all. Sharing code that few people de facto can use shouldn't give you much credit, or at least not as much as the alternative. Unless it is, as someone suggested, about signaling.

It's probably easier to just keep a set of diffs.

Even maintaining a patch against a rapidly changing codebase takes work, and then you've got to make builds. This is why people contribute changes upstream rather than everyone maintaining their own modified version of every application and library.

[edit: Moved here: https://news.ycombinator.com/item?id=16994356]