The current Stable release of NSS is 3.64, which was released on 15 April 2021. (NSS 3.64 release notes)
The current ESR releases of NSS are 3.44.4 (NSS 3.44.4 release notes), intended for Firefox ESR 68, which was released on 19 May 2020, and 3.53.1 (NSS 3.53.1 release notes), intended for Firefox ESR 78, which was released on 16 June 2020.
Release planning is done on the Mozilla wiki: NSS:Release Versions.