The suffix -eur in modern French typically gives feminine nouns: erreur, ferveur, torpeur, fureur. (Confusingly there's also -(a)teur which gives masculine nouns, but it seems etymologically separate).
The original suffix in Latin is -or, with words like error, fervor, torpor, and furor all being masculine. In both Spanish (-or) and Italian (-ore), the suffix is also masculine.
Do we know when and through what process this suffix became feminine in French?