Firefox-nightly fails on flake pure eval mode

I am currently using nix flake, which requires pure-eval mode. However, firefox-nightly-bin in nixpkgs-mozilla is currently “impure”. So I am unable to install firefox nightly with an error thrown out:

in pure evaluation mode, 'fetchurl' requires a 'sha256' argument

How do I install firefox-nightly with flake presented? Any solution here?

I built and have been using this:

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