Tails includes an "Unsafe Browser" which connects in the clear. So on top of a Firefox exploit, you would need another exploit to launch that browser or an exploit to escalate to root and tamper with the firewall rules. At least one Tails user has been successfully targeted like this ("an exploit taking advantage of a flaw in Tails’ video player to reveal the real IP address of the person viewing the video").[1] With Whonix, even an attacker with root would not be able to make a non-Tor connection because the firewall runs on a separate virtual machine.

[1] https://www.vice.com/en/article/v7gd9b/facebook-helped-fbi-h...