The feminine forms of some adjectives are based on an archaic masculine form.  This archaic masculine form is still used before masculine nouns that begin with a vowel or silent 'h'.

Masculine beau nouveau vieux fou mou
Archaic Masculine bel nouvel vieil fol mol
Feminine belle nouvelle vieille folle molle
beautiful new old crazy soft

Normally, adjectives ending in -x replace the -x with -se to form the feminine. 



Masculine Feminine
amoureux amoureuse in love
courageux courageuse courageous
curieux curieuse curious
ennuyeux ennuyeuse boring

Masculine Feminine
actif active active
bref brève brief

acteur actrice acting
conservateur conservatrice conservative
créateur créatrice creative

in:  http://www.languageguide.org/francais/grammar/adjectives/odd.html