toy fox terrier: agression. need help!

Have you been to obedience class and had a trainer evaluate her behavior?  That would probably be a smart place to start.  

It can happen for so many different reasons, because something that happened in the past, because she feels you belong to her, etc

NILIF (google) is a good program easy to follow in case your dog is being protective or thinks you belong to her, you can try that and if not then you need to re direct the behavior in some way, it seems that she listens when you say no so maybe you need to be more firm about it and make her take what you are saying seriously, if you give up after just one try she knows already that you wont do it for more than 5 seconds 
 
Also you have to check if by being nervous everytime you see a stranger coming you might be sending signs to the dog to start being aggressive

Was this dog taken to a puppy class or socialized as a puppy?

Oftentimes, dogs are more fearful of men than they are of women or children.  Men are bigger and louder. []  If you think she is aggressing out of fear, try getting a copy of "Scaredy Dog" by Ali Brown.  Even if she is not fearful, some of the techniques (for focusing your dog's attention on you) are valuable anyway.  My advice would be to get her to a "Growlies" or "Feisty Fido" type class, or have a trainer evaluate her to see if she could fit into a regular basic class.  Avoid aggressing against her yourself (no choke chains, leash pops, etc.) and find a positive trainer who can help you train, not punish the dog.  With reactivity and aggression, punishment or a tense handler often makes matters worse.  Don't scold her, instead just go the other way until she calms, then ask her to "sit" and reward her for keeping her attention on you, with multiple treats if necessary.