From the how many times can you celebrate a birthday files:
Apparently Mozilla differentiates between the day the project was created and the day that source code became available.
Today is a special day.
March 31, 1998 is the date that Mozilla was officially launched.
It’s the date the first Mozilla code became publicly available under
the terms of an official open source license and a governing body for
the project – the Mozilla Organization – began its public work
While I’m all for celebrating birthdays — generally speaking I prefer to celebrate them once per year. Then again 10 is a significant number and it does mark the beginning of the second decade of Mozilla as an open source project — and the second decade of Open Source itself. The great success that Mozilla has enjoyed in recent years is proof positive that the open source model can scale and innovate. Happy Birthday Mozilla !