I've read that a few times now and I'm trying to decide what I think they are actually asking for. It's a bit too vague to pin down.

Mozilla does not need a world wide royalty free licence to use anyone's content if the browser is just a pipe through which connect to the web.

So what exactly are they going to use that licence to your content for and how long are they going to retain it?

Mozilla does a lot of talking about creepy behaviour. Maybe it's just the wording of this but so far it feels a bit off to me.