That would be the problem if 1. firefox only had a _literal_ handful of users (rather than 100s of millions), or 2. if any browser was _that_ good at anti-finge

That would be the problem if 1. firefox only had a _literal_ handful of users (rather than 100s of millions), or 2. if any browser was _that_ good at anti-fingerprinting. But at the moment which browser you use is nowhere near as powerful in identifying you as the rest of the huge fingerprinting surface available. Fingerprinting currently gives pretty much everyone a unique fingerprint in almost every browser, so even if only 100 people managed to share a fingerprint, that would already be a big success. Firefoxes relatively low usage share doesn't currently add up to any real significance in the fingerprinting calculus.