I am currently building the latest Firefox updates and I have noticed that they have upgraded the CentOS-7 Firefox from the ESR tree (45.8) to the mainline tree (Currently firefox-52.0). They have left EL5 and EL6 at the ESR level (45.8.0-2).
EL7: https://rhn.redhat.com/errata/RHSA-2017-0461.html
EL5 and EL6: https://rhn.redhat.com/errata/RHSA-2017-0459.html
As stated above, I am currently building and testing these, so they are not yet released .. just preparing people for the changes.
Thanks, Johnny Hughes