Well as I don't use the Avast Secure Browser I can't say for certain, however if I want to check what version of Firefox is installed, I just look at the Help - About Firefox.
If you run the Avast Secure Browser and check its settings and see if there is the same option.
If you look in the link I gave you that will tell you what is the latest version is and the question you are asking may well have been discussed.