Obligatory IANAL, but I did just finish working with my attorneys on our new privacy policy, so this is fresh in mind.
"Mozilla explained it used specific legal terms -- "nonexclusive," "royalty-free," and "worldwide" -- because Firefox is free, available globally, and allows users to maintain control of their own data."
This, children, is what we call lying. It is where you say a thing that isn't true.
nonexclusive = use and sharing of data is not limited to Mozilla
royalty-free = the user who owns/generates the data doesn't get paid (The intellectual property owner)
worldwide = we can send your data anywhere and into any regulatory regime and you can't do anything about it.
I don't know or care if Mozilla is using this for AI, but technically speaking, if you use Firefox to upload content you created to YouTube, Mozilla has a non-exclusive, royalty-free, worldwide license to do anything they want to with your intellectual property.
Keep in mind Mozilla is a CORPORATION. They may or may not do any given thing, but they retain these rights and licenses meaning that if EvilCorp buys Mozilla at the impending bankruptcy, your personal licenses to everything you do or transmit in Mozilla goes with it.