Using Operator Mono with Fira Code ligatures in Atom.

1. Open your Atom's Stylesheet
   

2. Put this css

atom-text-editor {
  text-rendering: optimizeLegibility;
  -webkit-font-smoothing: antialiased;
  font-family: "Operator Mono";
  font-weight: 500;
  line-height: 1.7;
}

For Atom version < 1.13.x

atom-text-editor::shadow {
  .storage.type.function.arrow,
  .keyword.operator:not(.accessor),
  .punctuation.definition.entity.css {
    font-family: 'Fira Code';
  }

  .string.quoted,
  .string.regexp {
    -webkit-font-feature-settings: "liga" off, "calt" off;
  }
}

For Atom version >= 1.13.x

atom-text-editor.editor {
  .syntax--storage.syntax--type.syntax--function.syntax--arrow,
  .syntax--keyword.syntax--operator:not(.accessor),
  .syntax--punctuation.syntax--definition {
    font-family: "Fira Code";
  }

  .syntax--string.syntax--quoted,
  .syntax--string.syntax--regexp {
    -webkit-font-feature-settings: "liga" off, "calt" off;
  }
}

Save file and it's done.

Works fine with Javascript: