Hey John, re-posting my response to a similar conversation:
This pops up periodically. Per this thread on the Mozilla forums , Mozilla “stopped putting minor version numbers in the user agent string after Firefox 10”. This is problematic for Extended Support Release (ESR) detection, because the user’s browser no longer publishes enough information to allow Duo to reliably determine whether the browser is a fully-patched ESR version.
I believe this has been fixed in previous ESR versions. It may be addressed in the one you’re referencing.