It does not seem to be part of the official Debian repositories; searching for librewolf returns no results: https://packages.debian.org/search?suite=default&se

It does not seem to be part of the official Debian repositories; searching for librewolf returns no results: https://packages.debian.org/search?suite=default&section=all&arch=any&searchon=names&keywords=librewolf https://packages.debian.org/search?suite=default&section=all... They have their own external repository indexed by extrepo, but "no warranty is made by the Debian project as to the security or quality of the software in these third-party repositories."