Things that happen, are done, or are achieved for the very first time, or people who are the first to accomplish something.
From 'first' (Old English 'fyrst'), meaning 'before all others.' The plural 'firsts' evolved to refer to the historical achievements of being first or the people who achieve them.
History remembers 'firsts' obsessively—the first person to climb Everest, the first to run a four-minute mile, the first to walk on the moon. But every single person reading this has a dozen personal 'firsts' they're proud of that nobody will remember, yet they changed the shape of their own lives.
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