There's no fair way to measure that.
If I write a 10 line patch while watching TV one night, and then a Netscape employee spends an hour reading it, applying it to her tree and then testing it, gets r/sr= and checks it in to the next version of Mozilla, what %age work did I do ?
If we count lines of code that was 100% me, if we look at time spent it was 95% Netscape, it we measure comments in Bugzilla it might be an 80:20 split. Statistics are only useful if you understand WHY as well as WHAT and HOW MANY.
As a guestimate I would say that at least 90% of the resources plowed into Mozilla are coming from AOL/Time Warner/Netscape/Pepsi or whatever that corporation is these days. Don't underestimate the value of the other commercial interests and volunteers though, they are essential to the Mozilla project.
I don't know the precise reasons for the "special" Netscape rules, maybe someone more responsible can enlighten you. I do know that mcom.com internal machines (like sweetlou) are for internal Netscape use only. If you see one mentioned where it seems Mozilla.org would be more appropriate then politely ask for the patch/ data/ whatever to be moved somewhere public.