Not related to 'one' — it's the superlative of 'fore,' from PIE *per- (forward), cousin to Latin 'primus' and Greek 'protos.'
Coming before all others in time, order, or importance; earliest in a sequence.
From Old English 'fyrst' or 'fyrest,' a superlative form of 'fore' (before), from Proto-Germanic *furista (foremost), from PIE *per- (forward, through, before), one of the most prolific roots in any language. The structure is notable: 'first' is etymologically a superlative ('most before'), paralleling how 'foremost' combines 'fore' + '-most.' The PIE root *per- generated an extraordinary range of English vocabulary: 'for,' 'fore,' 'before,' 'far,' 'from,' 'forth,' 'further,' 'former,' 'foremost,' 'prince' (via Latin 'princeps,' literally 'first-
German 'Fürst' (prince, sovereign) is the same word as English 'first' — both from Proto-Germanic *furistaz. The German word preserves the ancient connection between being first and being a ruler: the prince is literally 'the foremost one.' English lost this political meaning, keeping only