For ESR (the primary but not the only version I run), the most recent one available as of yesterday (4/21/17) was 4580. Today, I see this has somehow jumped to 5210 (?), however I am still getting Mozilla messages to update ESR to v. 4590. So I'm a bit confused. Standard portable has just released v. 53.
While I'm at it, I'd like to know if the policy on allowing user exceptions for "flagged" extensions we choose to keep on running has changed, or is due to change any time soon. If it has or will, I plan to stay with ESR 4580 for as long as I can. As I have noted before, retaining certain functionality that I want trumps security or whatever other reasons for policy change happens to motivate the powers-that-be in FF development.