My preferred online dictionary for French is Word Reference (http://www.wordreference.com/). It has English to French translations as well as French to English translations. It doesn’t have Portuguese-French, but it does have Portuguese-English, so you might be able to translate a word into English and then French if Portuguese is your native language.

Doing a brief Google search gave me Ultralingua, Babylon and Reverso – you can choose to translate between any two languages you want here (I would post these links but stackexchange says I haven’t got enough reputation). It should be easy enough to Google these two.

My personal favourite for looking up words in context is Linguee (http://www.linguee.com/english-french/?moreLanguages=1#moreLanguages) which allows you to look up words used in context in one language (includes English, French and Portuguese) and see the direction translation of the word in context in the other language. This site uses content from multilingual websites that have the same content translated into different languages.

Hope this helps!