There is less space between Wikipedia's model and open source development than you seem to think. Forking Wikipedia is comparable to forking a large software project. Actually probably simpler. One significant difference is that because the version control systems for software are more sophisticated it is easier to merge changes between forked versions. With Wikipedia it is easy to fork but hard to keep parts of your new version in sync with changes to the original (should you want to).

Notwithstanding the above technical issue, it seems to me that the big difference is that a big part of Wikipedia is that it is THE Wikipedia. It is the destination.

Actually, Wikipedia is way, way more open than an open source project, come to think of it. No serious open source software project allows anyone to anonymously edit their source code. Usually you have to submit some patches for review before you're allowed to check in code.

Also, Firefox isn't really a fork of Mozilla. "Forking" doesn't so much refer to a different application springing from a common codebase, as two divergent versions of essentially the same software (like GNU Emacs and XEmacs).