If there was a "backdoor" it would hardly be mentioned on the website, that's ridiculous.
There was (possibly) a firefox exploit, these aren't very uncommon and this is why you should run up to date software. Browsers are large, complex code bases that try to open every type of file imaginable, they have a huge attack surface. Regardless, we have yet to see any evidence that the FBI (or similar LEA) has ever deployed an exploit against what was the latest tor browser at the time.
All previous examples, except for CMU's traffic confirmation attack relied on users enabling flash or running out of date software that had known vulnerabilities in it. This means that users were either ignoring warnings about updates, using their own software or disabling things put in place to protect them from exactly the kind of thing that stung them.
The FBIs own numbers don't add up, they probably have some exploit, but given their claims of the size of the user base of the forums and the number of people they managed to exploit (a tiny fraction of that user base), it seems unlikely that there is a backdoor or even a currently useful exploit.
This doesn't mean that it's impossible to exploit firefox or tor. You should of course take reasonable precautions. These would involve steps like isolating your client application by using sandboxing (VMs, namespaces, containers, seccomp, ...) and access control mechanisms (apparmor, RBAC, ...) to limit the capabilities of tor, firefox, or any other clients you have that might present an "attack surface", this reduces the impact of any such exploit and potentially stops it dead and importantly increases the risk that the exploit will be spotted. Further more, use of the Tor Browser's Security Slider will disable large portions of the potential attack surface.
Secure operating system projects like Tails, Qubes, Subgraph (Alpha), and Whonix all attempt to provide some of these features.
The FBI are currently taking extreme measures to ensure that the do not have to reveal their NIT software and it's deployment mechanism. Their "magneto" exploit was captured, analyzed and reverse engineered. We have a good understanding of what it is and how it operated and more importantly, from the perspective of the FBI, it's limitations.
The FBI has a long history of using misdirection, lies, intimidation and assassination to discourage political dissent and I'd be careful about believing what their press statements say or journalists parrot (414 becomes 27?).
Edit: Add more politics, correct some typos, add more references.
