A dog with rabies will froth at the mouth (saliva bubbles at the mouth). An angry dog sometimes will froth at the mouth. A tired, hot horse will foam at the mouth. In movies, people who have been poisoned are sometimes depicted with foam at the mouth. I have never used these expressions. I suppose they could be used metaphorically for very angry people.

People froth at the mouth (metaphorically) when they are angry. Horses foam (literally) when they are hot

I've never heard these used before in slang.

Never. I don't. . People will use this to mean, "calm down", "Don't overreact".