French feminine nouns ending in eur are a different group from the masculine ones. They almost never come from English words for people or machines. Instead, they are words for qualities, feelings, or abstract ideas. The English cousins of these words often end in -or (like color, horror) but sometimes in -ness or other endings.

These nouns are always feminine. Here is a list of common ones.

• la chaleur (heat, warmth)
• la couleur (color)
• la douceur (softness, sweetness, gentleness)
• la faveur (favor)
• la fleur (flower)
• la frayeur (fright, fear)
• la froideur (coldness)
• la fureur (fury, rage)
• la grandeur (size, greatness)
• la grosseur (size, bigness)
• la hauteur (height, arrogance)
• l’ horreur (horror)
• la largeur (width)
• la lenteur (slowness)
• la lourdeur (heaviness)
• la malheur (misfortune, sorrow, this one is an exception: le malheur is masculine)
• la moiteur (dampness)
• la noirceur (blackness, darkness)
• la odeur (smell, odor)
• la pâleur (paleness)
• la peine (sorrow, trouble, not an eur word)
• la pesanteur (heaviness, gravity)
• la peur (fear)
• la profondeur (depth)
• la puanteur (stench)
• la rapidité (speed, not an eur word)
• la rondeur (roundness)
• la rougeur (redness)
• la rugosité (roughness, not an eur word)
• la stupeur (stupor, astonishment)
• la sueur (sweat)
• la tiédeur (lukewarmness)
• la torpeur (torpor)
• la valeur (value)
• la verdeur (greenness, sharpness)
• la vigueur (vigor)

Some words for feelings or qualities can be masculine if they come from a different root or have a different use. Le bonheur (happiness) and le malheur (sorrow) are two important ones. L’honneur (honor) is also masculine.

You will not find many feminine eur words for jobs. When a job word like acteur (actor) becomes feminine, it usually changes its ending. It becomes actrice. A dancer (danseur) becomes danseuse. The feminine forms use -trice or -euse, not just -eur.

Summary

Feminine nouns ending in eur are for qualities and abstract things, not for people. They come from Latin words and have English counterparts often ending in -or. The gender is a firm rule: these words are feminine, with only a few common exceptions like le bonheur.