[Of course, if a lawyer/firm started taking ideas they would quickly get a bad rep and it would probably damage their business more than they would gain from the idea. But just wondering anyways.]
You should worry more about your mom/dad/brother/girlfriend/boyfriend/best friend saying something to someone than worry about a lawyer stealing anything.
My lawyer doesn't even allow anyone to look at his laptop's screen and he never mentions his other clients.
Everyone has great ideas that they can't implement. That's why I learned to program in the first place.