In a recent Tor security advisory, Roger Dingledine writes:
I am less concerned with linkability/fingerprinting than I am with security, i.e. maintaining the anonymity of my IP address by not getting owned.
The security advisory quoted above also touches on stuff Tor users can do at the OS level to increase security. This could involve using Tails or possibly a VM-based approach (examples are Qubes or Whonix). There are plenty of other OS options, like OpenBSD, Hardened Gentoo, and so on, each with its pros and cons.
Let's just focus on Firefox, though - regardless of OS.
Even then, this is a pretty broad question. Maybe it should be split into more than one question?