Release notes include a link to a list of new contributors and their contributions. Never noticed that before.

https://blog.mozilla.org/community/2014/02/03/firefox-27-new...

Looked up some past numbers:

  Firefox 24: 43 new contributors, 32 volunteers
  Firefox 25: 51 new contributors, 39 volunteers
  Firefox 26: 43 new contributors, 27 volunteers
  Firefox 27: 41 new contributors, 35 volunteers

I think Mozilla considers its employees and the employees of known major contributing companies to not be volunteers.