Learn what "firstly don't judge a book by its cover" means, when people say it, and how to use it naturally in English.

It means you should not form an opinion based only on appearance.
From I Think My Wife Wants To Kill Me S2, Episode 40
He reminds someone not to judge others too quickly.
Use this as a general warning or reminder in everyday conversation. It is very common, but the 'firstly' makes this version sound a bit unnatural.
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