Basically the title.
I have Nixos running on a server that runs completely headless and while playing around today, I noticed that the rebuild takes longer than expected and apparently that's because firefox gets compiled.
Now, I don't have any GUI installed, and even if I had, I don't see a reason to compile FF from source.
My packages are just Jellyfin, Samba, Gitea, Nextcloud, virt-manager. None of these should depend on FF.
I solved it using nix-tree, see the other comment.