"Froth at the mouth" is an idiom that means to show extreme anger or excitement. However, there are plenty of other synonyms that can be used instead of this idiom. One could say that someone is "spitting fire", "fuming", "raging", "seething", or "furious". Other synonyms include "boiling over", "smouldering", "incensed", "enraged", and "livid". Each of these words can be used to describe someone who is intensely angry and emotionally charged. It's important to use the right word to express what you want to convey and to avoid overusing the same phrase repeatedly.