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Just a simple little nitpick:
On the Nix/NixOS section of install, you type
Alternatively, for non-NixOS users, you can spawn a temporary shell with Managarr available like so: nix-shell -p managarr
nix-shell -p managarr works just fine on NixOS proper. I use it all the time.
I’m using librewolf. I’m not sure it’s on your end. I think something else might be going on.