You’re correct that this isn’t exactly common practice, but it does generally make sense from the stance of legal protectionism. Mozilla just wants to make sure that no maliciously inclined user can try and argue that Mozilla didn’t have a right to use the content they put in the browser, when the browser could only do what that user wanted by putting that data in.

It’s not exactly necessary based on existing precedent, but to me at least, it seems like they’re preparing for situations where cases are brought and try to argue based on things that don’t have existing precedent. For instance, if you look at how new a lot of the arguments and defenses of AI are in court, if a user tried to argue that Mozilla didn’t have permission to send their data to an AI company if they highlighted some text and sent it to the AI sidebar, there’s a chance the court wouldn’t go based on existing precedent, and instead try to argue based on if Mozilla had a right to send that data, which this clause would then clearly, very objectively cover.

TLDR; I personally don’t think they really needed to do it, but it doesn’t functionally change anything about what they’re capable of doing compared to before.