It's only for links shared via WhatsApp for some reason. Not sure how they know you're sharing to WhatsApp... (Edit: firefox implements a custom share widget instead of the one provided by the OS, so they get a callback when the user selects the target)
OP is wrong about the "unique" part tho, I get the same URL as them.
Sharing is much quicker than copy/pasting manually, especially with direct share targets.
It might not be the end of the world, but Mozilla really should have made the "feature" opt-in, or at least give us a heads up.