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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?
I don't have the answer, but the Spanish, Italian and Portuguese descendents of these Latin nouns are masculine, whereas the French, Provençal and Romanian equivalents are feminine. Some of the French ones (ardeur, honneur, odeur) went back to being masculine in the 16th century, but the only ones that are masculine today are labeur and honneur
