Hi to all!

About 1,5 months ago we rescued a male Jack Russel Terrier. The vet says he's healthy and about 4 years old.
The first two weeks he was fine, very obedient and docile. Then he began to growl and bare his teeth at us when we were cleaning his feet after a regular walk. At the moment we're afraid to leave the house because as soon as you open the door and don't call him out, he rushes to the door barking fiercly and then howls behind the door. I have no idea if he can bite because so far, luckily, we've always managed to leave the house in time and shut the door. ))) Except this oddity, he's a great dog. Any ideas what we can do to stop this?
I have a different suggestion, as a positive reinforcement dog trainer.

From what you describe, this is what I think is going on and what you should do:

It sounds to me like he is fearful having his feet handled. This is quite normal for dogs. You need to slowly and kindly desensitize him to having his feet handled. (NOT traumatize him more by "enclose his paw in your hand and don't let go until he stops"!).

Get some bite-sized, yummy, chewy treats such as pieces of hot dog or cheese. Learn to use a clicker or marker word (see http://ahimsadogtraining.com/blog/clicker-training / or http://www.clickertraining.com / ).

Begin slowly by touching his foot. Mark the moment you touch and treat. You can slowly work up to picking up his feet, handling, and massaging them, clicking (marking) and treating all the way. This may take days or even weeks, but go at your dog's pace! Slow is better. For more information on this style of desentitization, see this story about Jack, an aggressive border collie: http://www.clickertraining.com/node/1650

The door aggression I don't quite understand without seeing your dog, but from what you've said I'd suggest teaching him to stay on a mat (also called "place training") when the door is opened. You can find more info on teaching a good place at: http://www.clickertraining.com/node/3308 and also from Victoria Stillwell, who is the fantastic trainer seen on It's Me or the Dog (Animal Planet).

Good luck!
It sounds to me like he's trying to assert his dominance as the pack leader. Rather than rushing out the door to avoid him misbehaving, you're going to have to take back control, and do it quickly before his behavior escalated into something worse.

When you are going out without him, make him sit quietly and stay. Let him know that YOU are the pack leader, and that he is going to stay behind whether he likes it or not.

The same goes for when you're wiping his feet after a walk. If he bares his teeth and growls, then enclose his paw in your hand and don't let go until he stops. Again ... let him know that whether he likes it or not, he IS going to have his feet cleaned.
Elizabeth Ann Craver(NOT traumatize him more by "enclose his paw in your hand and don't let go until he stops"!).
Then I guess dogs are just like humans and afraid of different things, for different reasons. When I tried using a clicker with my dog, he whimpered, cried and hid under the table. The sound of it traumatized him far more than me holding onto and/or touching his paw.
That's true that some dogs are frightened of the sound of a clicker at first, especially "box clickers" because they can be louder. You can muffle the sound of the clicker in your pocket as you desensitize him to the sound (click & treat, click & treat, click & treat...). He will soon associate the clicker with good things. Or you can not use a clicker at all and replace it with a marker word, such as "YES". (Yes & treat, yes & treat, yes & treat... until your marker is loaded and you can begin training).