You're correct that there is a tension here. This is why a few years ago (2012, iirc) Mozilla adopted an explicit policy that beliefs like that must stay outside of the project. See http://www.mozilla.org/en-US/about/governance/policies/parti... section (i).
This does not say that project participants cannot hold such beliefs; that is an untenable requirement for a project involving people from many countries and cultures. But such beliefs need to stay outside things like project mailing lists workplace conversations, and so forth.
This does not say that project participants cannot hold such beliefs; that is an untenable requirement for a project involving people from many countries and cultures. But such beliefs need to stay outside things like project mailing lists workplace conversations, and so forth.