Besides the point already made about firefox-esr being the default package for Firefox, here are a couple of other points. First, that also means there are 5749 installations of non-default Firefox compared to 231 of guix.
Secondly, popcon works on servers and headless systems without X or Wayland. Those systems still use a package manager, and sometimes several. A graphical web browser like Firefox is unlikely to be installed without a GUI. I’ve seen plenty of systems with Snap, Flatpak, or Nix installed besides the OS-related apt, aptible, apt-get, rpm, yum, dnf, and language-related tools like pip, EasyInstall, rubygems, Bundler, cpanm, npm, cargo, OPAM, Composer, go get, or sbt with no GUI.
Secondly, popcon works on servers and headless systems without X or Wayland. Those systems still use a package manager, and sometimes several. A graphical web browser like Firefox is unlikely to be installed without a GUI. I’ve seen plenty of systems with Snap, Flatpak, or Nix installed besides the OS-related apt, aptible, apt-get, rpm, yum, dnf, and language-related tools like pip, EasyInstall, rubygems, Bundler, cpanm, npm, cargo, OPAM, Composer, go get, or sbt with no GUI.