Librewolf comes packaged by my distro (GNU Guix) so that’s what I use. I’m sure most “privacy” or “hardened” Firefoxes are more or less interchangeable. The only one that’s really noteworthy is GNU IceCat, because it’s more focused on software-freedom and includes the LibreJS addon, but I switched to Librewolf once it was packaged for Guix.
GNU/Linux != Linux
Linux is a kernel
GNU/Linux is the GNU userland (tools and libraries) combined with the Linux kernel to form a complete operating system
Android is Linux but not GNU. So are Alpine, postmarketOS, and others I can’t think of
Linux is to an operating system as bread is to a sandwich… an essential component, but a slice of bread by itself does not make a sandwich make