Is silence towards someone who insults one's mother in front of people foolishness, and does that fall under disbelief, considering that some messengers and companions remained silent towards those who insulted them?

We advise the questioner to ignore obsessive thoughts, as that is the most beneficial cure for them. A Muslim is required to restrain his tongue except for the remembrance of Allah or speech that is not sinful, because his words are part of his deeds and he will be held accountable for them. Cursing a Muslim is a transgression (fusuq) and fighting him is disbelief (kufr). It is permissible for one who has been cursed to retaliate in kind, unless the curse is directed at one's parents; in that case, it is not permissible to retaliate in kind. Rather, one must denounce the act and warn the perpetrator of its gravity. What the questioner mentioned is merely an obsessive thought and does not lead to disbelief.