Title is a complete lie/misleading. They get a license, not ownership.
Let's dissect what it actually says, and we do it backwards, because given the discussion around this subject it seems like people space out or have their mind clouded by outrage before they get to the end of the sentence:
> help you [do things] as you indicate with your use of Firefox.
So this already only covers things that you indicate you want to do with your use of Firefox. Meaning that if you hit some button, Mozilla now has a license to process the data they need to make that button work and nothing more. That means unless you give them additional permission somewhere, they can't, for example, also store and process that information to train some AI model or whatever. All they're allowed to use it for is making whatever you interacted with work. Seems pretty reasonable.
> to help you navigate, experience, and interact with online content
This further narrows the scope to websites and such you interact with (online content). It also says that license only covers "helping" you with these things. The part we looked at previously narrows this to your intent.
> you hereby grant us a nonexclusive, royalty-free, worldwide license
So just a license. No transfer of ownership is happening.
> When you upload or input information through Firefox
Note that this says "through". They're clearly only trying to cover their butt as an intermediary by obtaining a license to process your information to act as such an intermediary. Explicitly nothing more.
Putting it back together we get:
> When you upload or input information through Firefox, you hereby grant us a nonexclusive, royalty-free, worldwide license to use that information to help you navigate, experience, and interact with online content as you indicate with your use of Firefox.
Important part in cursive.
So broadly what is the license for?
> license [..] to help you [..] as you indicate