If Linux is just the kernel then Android and Ubuntu are equally Linux.
It is and they are. These are demonstrable facts.
I have no problem with referring to the family of Linux based operating systems collectively as Linux (with GNU/Linux being a subfamily of such), however, I firmly believe that the mythical concept of “real Linux” where some Linuxes aren’t really Linux is what creates the confusion. I would rather use other terms, like POSIX, Unix(like), and FreeDesktop to refer to so-called “real Linux” (with the caveat that they also include BSDs and the like - but I include these as part of the free desktop operating system spectrum, as most so-called Linux apps also run here. I don’t place special importance on the kernel because it is technically the furthest thing away from the user experience).
(Android being Linux isn’t a mere technicality - it means you can get a full terminal environment with a package manager and “Linux apps” and even run a full desktop environment if you really want)
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.