"firstojn" meaning in Espéranto

{
  "categories": [
    {
      "kind": "other",
      "name": "Formes de noms communs en espéranto",
      "parents": [],
      "source": "w"
    },
    {
      "kind": "other",
      "name": "Espéranto",
      "orig": "espéranto",
      "parents": [],
      "source": "w"
    }
  ],
  "forms": [
    {
      "form": "firsto",
      "ipas": [
        "\\ˈfir.sto\\"
      ],
      "tags": [
        "singular",
        "nominative"
      ]
    },
    {
      "form": "firstoj",
      "ipas": [
        "\\ˈfir.stoj\\"
      ],
      "tags": [
        "plural",
        "nominative"
      ]
    },
    {
      "form": "firston",
      "ipas": [
        "\\ˈfir.ston\\"
      ],
      "tags": [
        "singular",
        "accusative"
      ]
    }
  ],
  "lang": "Espéranto",
  "lang_code": "eo",
  "pos": "noun",
  "pos_title": "Forme de nom commun",
  "senses": [
    {
      "form_of": [
        {
          "word": "firsto"
        }
      ],
      "glosses": [
        "Accusatif pluriel de firsto."
      ],
      "id": "fr-firstojn-eo-noun-tjK7y~xI"
    }
  ],
  "sounds": [
    {
      "ipa": "\\ˈfir.stojn\\"
    }
  ],
  "tags": [
    "form-of"
  ],
  "word": "firstojn"
}

{
  "categories": [
    "Formes de noms communs en espéranto",
    "espéranto"
  ],
  "forms": [
    {
      "form": "firsto",
      "ipas": [
        "\\ˈfir.sto\\"
      ],
      "tags": [
        "singular",
        "nominative"
      ]
    },
    {
      "form": "firstoj",
      "ipas": [
        "\\ˈfir.stoj\\"
      ],
      "tags": [
        "plural",
        "nominative"
      ]
    },
    {
      "form": "firston",
      "ipas": [
        "\\ˈfir.ston\\"
      ],
      "tags": [
        "singular",
        "accusative"
      ]
    }
  ],
  "lang": "Espéranto",
  "lang_code": "eo",
  "pos": "noun",
  "pos_title": "Forme de nom commun",
  "senses": [
    {
      "form_of": [
        {
          "word": "firsto"
        }
      ],
      "glosses": [
        "Accusatif pluriel de firsto."
      ]
    }
  ],
  "sounds": [
    {
      "ipa": "\\ˈfir.stojn\\"
    }
  ],
  "tags": [
    "form-of"
  ],
  "word": "firstojn"
}