Traduzione - Francese-Portoghese brasiliano - Vous êtes mon amie pour toute la vie !!!!!!

Isn't 'mon' male and 'amie' female?

You are right, Casper, but French people don't use the feminine "ma" before words that start with a vowel. In this case, they use the masculine one: "mon amie" and not *"ma amie", to prevent the colliding of two identical vowels. This is called "euphonics".

The same thing occurs in Spanish with the singular definite article: they say "el agua", "el águila", even being both nouns feminine.

So it would be 'amiga', wouldn't?

It is, indeed. I don't know why French people concerns so much writing differently what is pronounced the same way... :/

But in Spanish it doesn't apply to every feminine noun beginning by an "a", it only applies if the "a" is stressed.
la aMIga
el HAda
una aLONdra
un ANcla