Do you think that the latest version or Firefox has better virus scanning than a version that's 4 years or so old ? I'm fine with the 56.0, as long as it doesn't put my PC at risk.
Quoting Mozilla:
"Up-to-date browsers protect you from viruses, security breaches and hacks. Older versions of browsers may be vulnerable to attacks and security holes. Firefox engineers have been known to ship a security update within a day of learning of a vulnerability."