When using Tor Browser I see that the Tor Browser and Firefox versions differ.
The current version of Tor Browser (based on firefox) is 45.7.0 The current version of Firefox is 51.0.1
Why doesn't Tor Browser use the same version of Firefox? Doesn't this pose a security risk?
The Tor Browser uses a long-term supported version of Firefox, to be sure its vetted from the security bugs and to provide the most stable and expectable behavior possible.
Tor Browser is a Firefox ESR(extended support release) tweaked in a way that are supposed to improve your security and anonymity. Because it's a long-term supported release - it's obvious that it's not the latest one, but security patches are supposed to be merged into it with the same speed as in a mainline one. Tor Browser does it's job well, but it's still a browser, so it's unable to protect you as wide as an isolated VM
