Fireglass Mentor’s ability cares whether an opponent lost life this turn, not how their life total changed. For example, an opponent who gained 2 life and lost 1 life in the same turn still lost life.
At the beginning of your second main phase, if an opponent lost life this turn, exile the top two cards of your library. Choose one of them. Until end of turn, you may play that card.
You pay all costs and follow all timing rules for cards played this way. For example, if the exiled card is a land card, you may play it only during your main phase while the stack is empty.
