您的JSON无效。将此更改为有效：
[{"id":"1","word":"cat","wordmeaning":"a mammal species","wordlength":"3"},
{"id":"2","word":"catapult","wordmeaning":"throwing device","wordlength":"8"},
{"id":"3","word":"cart","wordmeaning":"something to carry things in","wordlength":"4"}]
因此，这些是更改：
您可能要考虑不引用的值id和wordlength，因为这些特性似乎是数字。
这两个在线工具似乎可以正确验证JSON（或至少符合Firebase的期望）：
602392714
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