This may have been pointed out when it first came out, I don’t know. The Libre Software Engineering group at the Universidad Rey Juan Carlos in Madrid, Spain, has done some analysis on the CVS repositories of several large free software projects, including Mozilla. The analysis goes up to August 2003.
It’s split up primarly by modules, which isn’t very helpful because almost all of Mozilla is in a single module.
There’s a big list of all the people who’ve checked in anything to CVS. This bookmarklet for sorting tables numerically (thanks to Jan Moesen; Jesse’s bookmarklet sorts in alphabetical rather than numeric order) may well come in useful if you look at that.
The problem with this sort of thing is automated checkins and relicensing. For example, it appears that I’m around about sixth in the overall rankings for lines or files changed – when obviously the vast majority of that is not code contributions, but automated licence changes.