Mozilla's copy tone sticks to high-level tonal stuff, but does so in a refreshingly human way, and the rules section digs a bit deeper into details, and could be useful to anyone writing for the web.
Mozillians live all over the world. We have different roles. We do different jobs (often several at once). But one thing we have in common is that we're a group of human beings building products and technologies for other human beings.
Whenever a user — or anyone else, for that matter — reads a piece of Mozilla communication, they should feel like there's a person on the other end who recognizes that they are the same. They should feel like we care about them and how they spend their time online. That's not a gimmick or a piece of marketing, either. It's one of the principles Mozilla was founded on.
So what does that mean, practically speaking? It means that, yes, sometimes we'll use colloquial or conversational language. We'll use contractions and speak to people in a familiar way (sorry, couldn't think of another word that started with a C). We'll use humor to make people feel good. Or be serious when the situation warrants.
We should not, however, talk down to people or use overly technical language. It's OK to be clever, but not just for its own sake (or because we're too in love with our own words). We should avoid snark and cynicism. And we should never make people feel like we're smarter or better than they are. We should speak on their level, while being friendly and inclusive.
The point is, we never want our users to forget that there are real live people behind our products and our words. And when writing, we can't forget that, either. As our Firefox Brand Platform states, we should be unconventional, engaging, honest, smart and confident. If you remember that, you should be in good shape.