I have used more advanced fingerprint test pages, and subtle difference of browser is here.
It’s like
Original TB (one of 609**,6** machines)
TB+i2p+foxyproxy(one of 609**,5** machines)
So there is really change in fingerprinting.
Best way to stay stealth - use 2 instances of Tor Browser.
Original TB
and
TB for I2p - in this case foxyproxy not needed at all.